"He was alone. He wasn't watching the race," said Jeff Bauman, who walked slowly into federal court on two prosthetic legs. "I looked at him, and he just kind of looked down at me. I just thought it was odd."
Bauman later described Tamerlan Tsarnaev to the FBI from his hospital bed. Tsarnaev, 26, died in a gun battle with police days after the bombing.
Bauman testified at the trial of Tsarnaev's younger brother, Dzhokhar, who could get the death penalty if convicted of charges he helped carry out the 2013 bombings that killed three people and wounded more than 260.
Before testimony began Thursday, Tsarnaev's lawyers complained to the judge that the survivors' testimony from the previous day was too gruesome and should be limited.
Defence attorney David Bruck objected specifically to the testimony of three women who described their injuries in detail and what they saw in the aftermath of the attack. Bruck said that under the federal death penalty law, victim-impact testimony is supposed to be presented during the second phase of the trial, when the jury decides on the punishment.
Prosecutors denied that any of the survivors gave victim-impact testimony and said they merely described what they saw.
U.S. District Judge George O'Toole Jr. agreed with prosecutors and refused to limit survivors' testimony.
The trial opened on Wednesday, with Tsarnaev attorney Judy Clarke bluntly admitting to the jury that her client took part in the attack. But in a bid to save Tsarnaev's life, she argued that he was influenced by his older brother.
The first witness to testify Thursday was a policeman who was the first officer to reach 29-year-old Krystle Campbell, one of those killed.
Officer Frank Chiola said he ran across the street to help the victims as soon as he heard the explosions. As he reached Campbell and began doing chest compressions, he said, smoke came out of her mouth. He said she appeared to be in a lot of pain.