Boston (CNN) Bill Richard knew his son wasn't going to make it. But the father of three told his wife he couldn't stay by 8-year-old Martin 's side.

The boy's body was torn apart by an explosion near the finish line of the Boston Marathon. His skin had changed color. A crowd hovered over him, frantically trying to help, but he was dying.

Speaking from the witness stand at Dzhokhar Tsarnaev's trial on Thursday, Richard told jurors he was faced with a heartbreaking choice.

"I knew in my head that I needed to act quickly, or we might not only lose Martin," he said. "We might lose Jane, too."

Moments after the blast, Richard had stumbled toward Jane, his 7-year-old daughter. His pants and sneakers were torn apart. His legs felt like they were on fire. He could barely hear. And the air smelled "vile," he said, like gunpowder, sulfur and burned hair. But he soon realized the situation was much worse for his daughter.

"She tried to get up and she fell. That was when I noticed her leg," he said. "She didn't have it. It was blown off."

So Richard left one son to die near the marathon finish line, and shielded his other son's eyes from the carnage as they raced to the hospital, hoping that doctors could save his daughter's life.

"It was," Richard said Thursday, "the last time I saw my son alive -- barely."

Defense tries to stop testimony

Richard's description of the explosion's horrifying aftermath capped a day of dramatic testimony as survivors shared their stories in the second day of the high-profile trial.

Tsarnaev's attorneys admit that he carried out the 2013 attacks, which killed three people and injured more than 260 others at the marathon. A fourth person, an MIT police officer, was ambushed and killed in his patrol car three days after the bombings as Tsarnaev and his brother, Tamerlan, allegedly ran from police.

Tamerlan Tsarnaev, 26, was killed after a gunbattle with police. And now, his younger brother sits in court facing 30 federal charges related to the bombings. His attorneys say he was influenced by his slain brother to participate in the attacks.

They argued Thursday that now isn't the right time for jurors to hear the string of harrowing stories from survivors of the explosions, arguing that testimony should be part of a later phase of the trial, when jurors will decide what penalty Tsarnaev should face. But the judge sided with prosecutors, who argued the testimony was necessary to support their indictment.

On Thursday, jurors relived the moments after the marathon bombings through the eyes of some of the people most affected by the blasts.

Defense attorneys didn't ask them any questions.

'I could see my bone'

Jeff Bauman was suspicious as soon as he noticed a black backpack on the ground near the finish line.

"I thought it was weird," he said Thursday. "If you are at the airport, if you see any unattended luggage, you notify authorities."

But this was Boston, he thought, where stuff like that doesn't happen. Still, he told a friend they should move.

An explosion came two seconds later.

"I saw a flash, heard like three pops and I was on the ground," Bauman said from the witness stand Thursday. "At first I opened my eyes and saw the sky. The first thought was, that was a big firework."

Photos: Boston Marathon bombing evidence Photos: Boston Marathon bombing evidence A jury condemned Dzhokhar Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more in the 2013 Boston Marathon bombings. See photos that were released as evidence in his trial. Hide Caption 1 of 35 Photos: Boston Marathon bombing evidence This undated photo of a young Tsarnaev with his brother, Tamerlan, was shown by the defense in the sentencing phase of the trial. Tamerlan died after being shot by police and run over by a car driven by his brother in the massive manhunt that followed the bombings. Hide Caption 2 of 35 Photos: Boston Marathon bombing evidence Katie Russell met Tamerlan Tsarnaev at a nightclub and dropped out of college to marry him. Her mother, Judith Russell, testified that Tamerlan came between Katie and her family and that Katie became isolated. She eventually converted to Islam and changed her name to Karima Tsarnaeva. She was the breadwinner. But when company came for dinner, she cooked, served the men and then retired to another room. Hide Caption 3 of 35 Photos: Boston Marathon bombing evidence This collection of photos of Dzhokhar Tsarnaev in his wrestling days was introduced by the defense. Hide Caption 4 of 35 Photos: Boston Marathon bombing evidence This photo of Tamerlan Tsarnaev, recovered from his computer, was shown during the sentencing phase. Hide Caption 5 of 35 Photos: Boston Marathon bombing evidence This image shows victims' positions in the crowd prior to the Boston Marathon bombings on April 15, 2013. Hide Caption 6 of 35 Photos: Boston Marathon bombing evidence Tsarnaev "flips the bird" in a jail cell during his first arraignment on July 10, 2013. The image was presented to jurors in the sentencing phase of his trial. Hide Caption 7 of 35 Photos: Boston Marathon bombing evidence Tsarnaev poses in front of a black standard adopted by various militant Islamist groups in this Instagram photo that was entered as evidence. Hide Caption 8 of 35 Photos: Boston Marathon bombing evidence Prosecutors say Tsarnaev was a self-radicalized jihadist who pored over militant writings, including the article "How to Build a Bomb in the Kitchen of Your Mom." It was found on his laptop and other devices, part of a full-edition download of Inspire magazine, a glossy English-language propaganda tool put out by al Qaeda in the Arabian Peninsula. Hide Caption 9 of 35 Photos: Boston Marathon bombing evidence This Russian manual on how to fire a handgun was found in the apartment where Tsarnaev's brother, Tamerlan, lived. Tamerlan Tsarnaev was killed in a shootout with police in Watertown, Massachusetts, on April 19, 2013. Hide Caption 10 of 35 Photos: Boston Marathon bombing evidence This copy of The Sovereign, which calls itself the "newspaper of the resistance," was also found in Tamerlan Tsarnaev's apartment. Hide Caption 11 of 35 Photos: Boston Marathon bombing evidence Pictured here is a box of bullets found on a street after the shootout in Watertown. The brothers' fingerprints were on the box, prosecutors said. Hide Caption 12 of 35 Photos: Boston Marathon bombing evidence A pressure cooker was embedded in the side of a resident's Honda during the Watertown shootout. Hide Caption 13 of 35 Photos: Boston Marathon bombing evidence Photos of the Watertown shootout were entered into evidence. Neighbors came to their windows and then retreated. One grabbed his infant son and headed toward the back of his house with his wife. Another grabbed a camera and took photographs from an upstairs window. Hide Caption 14 of 35 Photos: Boston Marathon bombing evidence The Tsarnaevs had carjacked a Mercedes SUV in Watertown before the shootout. The vehicle was covered in bulletholes, and the rear window was shattered. Hide Caption 15 of 35 Photos: Boston Marathon bombing evidence This unexploded pipe bomb was found at the scene of the shootout between police and the Tsarnaev brothers in Watertown. Hide Caption 16 of 35 Photos: Boston Marathon bombing evidence Prosecutors said these boards were attached to the boat where police found Dzhokhar Tsarnaev hiding. A carved message reads, "Stop killing our innocent people and we will stop." Hide Caption 17 of 35 Photos: Boston Marathon bombing evidence Smashed phones and an ATM card owned by carjacking victim Dun Meng were in the yard where Dzhokhar Tsarnaev was found. Hide Caption 18 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this surveillance image shows Tsarnaev visiting an ATM hours before a police chase and chaotic shootout in which more than 200 rounds were fired. Hide Caption 19 of 35 Photos: Boston Marathon bombing evidence Another view of Tsarnaev's visit to the ATM. Hide Caption 20 of 35 Photos: Boston Marathon bombing evidence Exhibits related to the shooting death of MIT Officer Sean Collier were introduced to the jury on Wednesday, March 11. This image from the crime scene appears to show a bloody gun. Hide Caption 21 of 35 Photos: Boston Marathon bombing evidence This burned tank top and yellow hoodie belonged to bombing survivor Jessica Kensky. Hide Caption 22 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this Fox Racing logo was from one of the backpacks containing a bomb. Hide Caption 23 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this still image from surveillance video shows Tsarnaev in the UMass Dartmouth gym the day after the bombings. Hide Caption 24 of 35 Photos: Boston Marathon bombing evidence Prosecutors showed the jury photos of what they say are Tsarnaev's writings inside the boat he was captured in. Hide Caption 25 of 35 Photos: Boston Marathon bombing evidence This image is from a surveillance camera outside the Forum restaurant in Boston's Copley Square just after the bombing. Hide Caption 26 of 35 Photos: Boston Marathon bombing evidence Prosecutors presented two Twitter accounts linked to Tsarnaev that, they said, showed targeting the marathon had been on his mind for at least a year. One account, @J_tsar, contained 1,100 tweets and was the more mainstream of the two. On the day of the 2012 Boston Marathon, a tweet from the account read, "They will spend their money & they will regret it & they will be defeated." Hide Caption 27 of 35 Photos: Boston Marathon bombing evidence Prosecutors said the second Twitter account is evidence that Tsarnaev led a double life. By day, he was a slacker college sophomore. By night, he was a wannabe jihadist, posting on the account @Al_firdausiA. In one tweet, he urged people to listen to radical cleric Anwar al-Awlaki's lectures. "You will gain an unbelievable amount of knowledge," he said in March 2013, just weeks before the bombings. Prosecutors also allege in an indictment that Tsarnaev downloaded al-Awlaki's writings, calling him a "well-known al Qaeda propagandist." Al-Awlaki had been killed in a U.S. drone strike in 2011. Hide Caption 28 of 35 Photos: Boston Marathon bombing evidence Hide Caption 29 of 35 Photos: Boston Marathon bombing evidence A Boston Marathon bombing victim is tended to in the street. Hide Caption 30 of 35 Photos: Boston Marathon bombing evidence Victims at the finish line just after the bombing. Hide Caption 31 of 35 Photos: Boston Marathon bombing evidence Eight-year-old Martin Richard, the youngest victim , can be seen standing on the rail in the front row. Hide Caption 32 of 35 Photos: Boston Marathon bombing evidence A closer view of 8-year-old Martin Richard in the crowd before the bombing. Hide Caption 33 of 35 Photos: Boston Marathon bombing evidence Boston police tend to a wounded child. CNN has chosen not to show the young victim's face. Hide Caption 34 of 35 Photos: Boston Marathon bombing evidence Mayhem along Boylston Street. Hide Caption 35 of 35

Bauman's ears were ringing, and everything was muffled, but he heard the screams.

The first bomb had exploded.

"I looked down and saw my legs, and it was pure carnage," he told jurors. "I could see my bone."

Bauman testified about becoming aware of his injuries -- burns, wounds on his back, and his legs.

"I knew my legs were gone. I know that instantly," he said.

He kept repeating to himself, "This is messed up, this is messed up, this is messed up."

Then the second explosion.

"We are under attack," he thought to himself. All he wanted to do was call his mom.

Aiding at the scene

When Boston Police Officer Lauren Woods saw people running by, screaming, she ran against the grain, toward Boylston Street, the last leg on the marathon route.

Lu Lingzi, a graduate student at Boston University, was one of three people killed during the bombings at the Boston Marathon.

Lu was vomiting profusely, Woods recalled Thursday. Others were already performing CPR on her, and the officer attempted to clear Lu's airway.

"I smelled the residue of smoke," Woods said. "Smelled like fireworks, cannons."

Paramedics arrived, and eventually told Woods the young woman was not going to make it and they had to move on to other people.

Lu became one of the three fatalities at the scene.

A 'horror movie'

It smelled like gunpowder, Alan Hern told the jury, and "kind of felt like we were underwater."

Hern's wife -- who was pregnant -- was running the race. He found her uninjured, but hysterical from the explosions and ensuing chaos, he said. There were powder marks on her jacket.

Hern, a high school football coach, then went searching for his 11-year-old son, Aaron.

A figure covered in black soot and hair standing straight up caught Hern's eye, and he knew it was Aaron, he testified. His son's left thigh was cratered, mangled flesh and blood, Hern said.

"It was like something you'd see in a war movie," he said. "His eyebrows were singed and his hair was sticking straight up."

Another of Thursday's witnesses, Roseann Sdoia, also referred to it as something out of a movie.

Before she hit the ground from the blast, she said, it registered in her mind that she had lost her leg.

"When I looked down, my leg was tucked under me, but blood was pouring out," she told jurors.

In front of her lay a socked foot.

She started thinking -- Did I wear socks today? She decided she hadn't worn socks that day. It was someone else's foot.

"It was almost like I was starring in a horror movie," she said, "as was everybody else around me."

Doctors had to amputate her leg below the knee.

Devastating injuries

Traveling from the marathon finish line to the hospital, Bill Richard went from one terrifying scene to another.

"You know it's not going to be good when you see the look of horror on doctors' faces," he told the court Thursday. "Jane was devastatingly injured."

Doctors amputated her left leg below the knee and removed more than 20 shrapnel pieces from her body, he said.

Richard soon learned that, as he'd feared, Martin didn't survive the bombing.

An image taken from Facebook shows Martin Richard, the 8-year-old killed during the explostions at the Boston Marathon, holding a sign calling for peace.

His wife, Denise, was also hospitalized after the attack. She lost sight in one eye.

The father said his own injuries were much less severe than the wounds many other victims suffered. The ringing in his ears never stops, and he lost some of his hearing.

But Richard said he could still hear the lawyer questioning him at Thursday's trial. He can still hear music.

And the most important thing.

"I can still hear," he said, "the beautiful voices of my family."​