By Ann O'Neill

(CNN) -- An 8-year-boy who once urged people to "stop hurting each other," was literally blown apart by a homemade bomb as he watched the 2013 Boston Marathon with his family, jurors heard Monday.

Federal prosecutors rested their case with grisly testimony about how the bomb Dzhokar Tsarnaev placed near the marathon's finish line tore through the bodies of Martin Richard and Lingzi Lu, a 23-year-old grad student.

A second bomb placed by Tsarnaev's brother, Tamerlan, killed Krystle Campbell, a 29-year-old restaurant manager. Tamerlan Tsarnaev was killed in a gun battle with police three days after the bombing.

Martin's shredded, blood-stained clothes were displayed Monday in court.

No part of the boy's 69-pound body was left untouched by the explosion, said Henry Neilds, Boston's chief medical examiner. Neilds performed the autopsy and concluded that Martin bled to death after suffering blast injuries to his torso and extremities.

Several jurors cried openly in court, and the boy's parents sat silently in the audience. Bill Richard lowered his head and held his wife, Denise, close during the graphic testimony and display of gruesome photographs.

In his own testimony, Bill Richard had been unable to find the words to describe what he saw when he rushed to his son's side after the bomb went off:

"I saw a little boy who had his body severely damaged by an explosion," Richard told jurors earlier in the trial.

"This is difficult," he added, pausing to take a breath. "I just knew from what I saw that there was no chance. The color of his skin, and so on."

Neilds gave an official account of the wounds the boy's father found so hard to describe:

The bomb severed Martin's spinal cord and abdominal aorta; severed and exposed his lower intestines; ruptured his stomach; nicked or tore his liver, left kidney and adrenal gland; nearly tore off his left arm at the forearm; snapped a bone in his right leg; fractured and exposed his ribs; and bruised a lung.

Martin had third-degree burns on his back, buttock and left calf. His body was covered with scrapes, bruises and perforations from blast debris.

Many of the injuries were likely to result in death. But the most serious was his severed aorta. It is a major blood vessel, Neilds said, and Martin bled to death in minutes, if not seconds.

Assistant U.S. Attorney Nadine Pellegrini's last question for the government's 91st and final witness was this:

"How old was Martin Richards?"

"He was 8 years old," Neilds said.

And then the government rested.

Bodies riddled with injuries
Earlier Monday, a deputy from Neilds' office described the injuries to Lingzi Lu.

Katherine Lindstrom, who performed Lu's autopsy, said her thighs were perforated by debris spewed from the blast. Lindstrom said she found two shards of metal in Lu's legs, and a third, larger piece that slashed into her leather purse.

Metal and other debris, including pellets and small nails, were embedded in Lu's body. The gashes were "perforating," meaning they went completely though, Lindstrom said.

The skin from Lu's left thigh was torn away, and the femoral artery and vein were severed, causing her to bleed to death on the sidewalk "relatively quickly, from seconds to minutes."

Lindstrom was shown a series of graphic photos, and described the injuries she saw as "gaping," "slashes" and "cavities in the muscle."

"In her left thigh you see a large tear through her skin and muscle. It leaves a large hole that goes completely through her thigh."

Lingstrom testified that Lu likely remained conscious until she was near death.

A friend, Danling Zhou, told jurors she tried to calm Lu although she was seriously injured herself and holding her abdomen to keep her organs inside.

"I saw she's yelling. I can't hear, I can only see she's yelling."

She said Lu was lying on her back, waving her hand, and at first she was relieved because Lu did not seem badly hurt to her.

"It's hard to look at because everybody's injured. I look at her leg, and I think it's better than the man in front of me because he doesn't have a leg anymore. I think she is fine."

Last week, jurors heard from Jennifer Hammers, who performed the autopsy on Campbell. Hammers said Campbell's death was a homicide caused by blast injuries to the legs and torso. She said it probably took less than a minute for Campbell to bleed to death on the sidewalk.

Several jurors cried and others appeared upset on Thursday as they were shown graphic photos of Campbell, who was standing in front of Marathon Sports on Boylston Street near the finish line when the bomb placed by Tamerlan Tsarnaev exploded.

Photos and security surveillance videos introduced during the trial show Dzhokhar Tsarnaev sliding a backpack containing a pressure cooker bomb off his shoulder near the Forum restaurant, a block away. That explosion, 12 seconds after the first, killed Martin and Lu.

Hammers went on to explain how the homemade bombs maim and kill and to provide the gruesome details of the damage done to Campbell's body. The initial injuries came as a result of a shock wave from the blast: Campbell's tongue was bruised from being forced against her teeth, the femur of her left leg snapped and the bones in her left foot were crushed.

"The seams of her blue jeans were pressed into her body with such force, they caused bruising," Hammers said.

Other injuries were caused by bomb materials and debris sent flying through the air. Campbell's hair was singed, the skin on the back of her neck was abraded, and she had a large third-degree burn on her back. The top of her right arm was burned.

She had "big, gaping holes in her legs," Hammers said. The gashes were as large as 10 inches.

A pellet the size of a BB was embedded in her ear, and other BBs and metal fragments were found in wounds that penetrated her muscles. BBs also riddled her clothes.

"Miss Campbell passed away because she lost a significant amount of blood in a short period of time," Hammers said.

Beyond the clinical details of what happened to Campbell's body, jurors also have heard from a friend, who described what must have been the last minute of Campbell's life.

Karen Rand McWatters, whose leg was so mangled it had to be amputated, dragged herself to Campbell's side:

"I got as close as I could to her. There was so much chaos and so much screaming. For some reason, I got close to her head, and we put our faces together. I never really looked at Krystle's injuries. She very slowly said her legs hurt and we held hands. Very shortly after, her hand went limp and we never spoke again."

Prosecutors say the disturbing evidence is needed to prove several of the counts against Tsarnaev: using a weapon of mass destruction, bombing a public place and malicious destruction of property resulting in death.

Monday is the 15th day of testimony in the trial, which opened March 4 after lengthy jury selection.

Two phases of the trial
Jurors first will be asked to determine, perhaps by as early as the end of this week, whether Tsarnaev is guilty of 30 counts. Because 17 of those counts carry the death penalty as a possible punishment, a second phase of the trial will follow if the jury convicts him.

In the penalty phase, jurors will be asked to weigh aggravating factors, such as the heinousness of the crime, against mitigating factors, such as Tsarnaev's family history and his youth. He was 19 at the time of the bombings.

The defense, which began calling witnesses Monday afternoon, has argued that Tsarnaev, known to friends as Jahar, fell under the sway of his more extremist older brother after their parents moved back to Russia. Jahar Tsarnaev was flunking out of the University of Massachusetts and had lost his financial aid at the time of the bombings.

Prosecutors William Weinreb, Aloke Chakravarty, Nadine Pellegrini and Steve Mellin presented more than 90 witnesses who told the story of Tsarnaev's alleged scheme with his brother, Tamerlan, to build and detonate pressure cooker bombs as an act of jihad. The Russian-born brothers, Muslims of Chechen descent, allegedly sought to kill Americans at an iconic public event to retaliate against U.S. policies they believed harmed and oppressed Muslims abroad.

Prosecutors delved into Tsarnaev's text messages and Twitter posts and showed jurors militant material found in his laptop, phone and iPod. They included writings available online from top leaders of al Qaeda.

They used data mined from a GPS device and store receipts to trace the purchase of the pressure cookers, BBs and ammunition. Jurors saw photos of pressure cooker parts, fuses, Christmas lights and other bomb-making materials found in the Tsarnaev family's Cambridge apartment, where Tamerlan lived with his wife and child.

And they showed security surveillance videos of the brothers in the crowd near the finish line: In one, Dzhokhar Tsarnaev can be seen blending in with the crowd behind the Richard family for four minutes. He appears to slide a backpack off his shoulder near a tree and walk off, glancing over his shoulder. He broke into a run as the bomb went off.

After the surveillance photos were released to the public three days after the bombing, the brothers allegedly embarked on a desperate -- and deadly -- attempt to escape.

Jurors heard from carjacking victim Dun Meng and saw the brothers on convenience store surveillance video shortly before Meng's escape. He can be seen jumping out of his leased Mercedes SUV at a gas pump and running across the screen as Dzhokhar Tsarnaev casually strolls through the store, picking up an armload of snacks.

Prosecutors also used ballistic evidence to link the brothers to the shooting of a campus cop at the Massachusetts Institute of Technology and a gunbattle with police in Watertown.

Tamerlan Tsarnaev, 26, died as a result of that gunbattle. The defendant, allegedly attempting to run down police, instead ran over his brother in the stolen Mercedes.

Jurors also viewed a boat in which Dzhokhar Tsarnaev sought refuge during the ensuing manhunt, which put Boston under a "shelter in place" lockdown.

He used a pencil to scrawl what prosecutors called his "manifesto" on the sides of the boat. It was pocked with bullet holes and streaked with blood.

He wrote he was jealous that his brother had achieved paradise by dying like a holy warrior in the gunbattle with police. He asked God to make him a martyr, too.

His justification for his actions echoed themes of the militant material on his laptop:

"The U.S. Government is killing our innocent civilians but most of you already know that," he wrote. "Know you are fighting men who look into the barrel of your gun and see heaven, now how can you compete with that. We are promised victory and we will surely get it."

He wrote that he couldn't stand to see the U.S. government "go unpunished" for killing Muslims. "We Muslims are one body, you hurt one you hurt us all."

He ended with: "Now I don't like killing innocent people it is forbidden in Islam but due to said" -- the word was lost to a bullet hole -- "it is allowed."

Tsarnaev has pleaded not guilty, although his attorneys do not dispute that he participated in the bombings. Lead defense attorney Judy Clarke asked jurors in her opening statement to keep their minds open to an alternative explanation.

The defense began its case with two witnesses called to offer scenarios that differ from the version of events offered by FBI witnesses. One focused on the the defendant's Twitter posts a year before the marathon, including mundane matters such as whether he should sleep in or get breakfast. Another challenged the way the FBI used GPS points and store receipts to document the purchase of pressure cookers, BBs and ammunition -- allegedly by Tamerlan Tsarnaev.

By the end of the day, it became clear that the challenge for the defense will be to present a case that can somehow soften the harsh, unforgettable images jurors saw of ordinary people meeting a horrific end.
 
CNN's Alexandra Field and Aaron Cooper contributed to this report.