BOSTON (AP) — With Dzhokhar Tsarnaev seated at the defense table no more than 15 feet away Thursday, the father of an 8-year-old boy killed in the Boston Marathon bombing described the moment when he looked down at his son's pale, torn body and realized he wouldn't make it.

"I saw a little boy who had his body severely damaged by an explosion," Bill Richard told the jury, "and I just knew from what I saw that there was no chance, the color of his skin, and so on."

Martin Richard was one of three people killed in the bombing near the finish line of the race on April 15, 2013. The boy's younger sister, 6-year-old Jane, had her leg blown off, while their older brother, Henry, suffered minor injuries.

Their father, testifying at Tsarnaev's federal death penalty trial, spoke in a slow, halting voice but remained largely composed as he described the chaos and confusion.

He said he watched as Jane tried to get up, then fell down. He scooped her up in one arm and took Henry in the other and "tried to shield both of their eyes" as he took them away.

Richard said he ran back across the street twice to check on his wife, Denise, and Martin. It was then he realized the extent of his middle child's injuries, and turned his attention to those who still had a chance.

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

Tsarnaev, 21, showed no reaction to the testimony and appeared to look straight ahead, not making eye contact with Richard, who sat off to the side in the witness box.

Some of the women on the jury appeared to wince at times during his testimony. Spectators in the courtroom could be heard quietly crying, including Rebekah Gregory, who lost a leg in the bombing.

As Richard testified, the jury watched a video of the father rushing to help his children. Prosecutors paused the video frequently so that Richard could explain what was happening.

A prosecutor then circled a face in a photo — a young man in a white backward baseball cap — who could be seen just behind Jane and Martin, who were standing on a metal barricade, watching the race. It was Tsarnaev, shortly before the two pressure-cooker bombs went off.

Richard took the stand as federal prosecutors continued trying to drive home the horror of the attack in such graphic detail that Tsarnaev's lawyers objected — and were overruled.

Earlier in the day, Roseann Sdoia testified that she saw two flashes of white light at her feet near the finish line, looked down, and for a split second thought to herself: I'm wearing strappy sandals.

She quickly realized she was looking at her foot dangling from her mangled leg.

"Someone came running over to me and told me I had to get out of there. I told them I couldn't get up. I didn't have a leg," a sobbing Sdoia told the jury.

She walked to the witness stand on an artificial leg, plainly visible below the hemline of her skirt.

Sdoia, who was at the race as a spectator, said she saw wounded people all around her, including someone covered with soot, dazed and "walking around like a zombie."

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

Prosecutors also showed the jury a grisly photo of her shredded leg.

Tsarnaev's lawyer has admitted the former college student took part in the bombings. But in a bid to save Tsarnaev from a death sentence, she argued that he was influenced by his older brother, Tamerlan, who was killed in a getaway attempt days after the bombing.

Also Thursday, Jeff Bauman — who lost both legs in the attack and was photographed being wheeled away that day in one of the most widely seen images of the tragedy — testified that he locked eyes with one of the bombers shortly before the twin blasts.

"He was alone. He wasn't watching the race," said Bauman, who walked slowly into court on two prosthetic legs. "I looked at him, and he just kind of looked down at me. I just thought it was odd."

Later, from his hospital bed, Bauman remembered the man's face clearly enough to give the FBI a description of someone authorities say turned out to be Tamerlan Tsarnaev.

Before testimony began Thursday, Tsarnaev's lawyers complained to the judge that the survivors' testimony from the previous day was too gruesome and that such accounts should be limited.

U.S. District Judge George O'Toole Jr. refused.