Gilberto Junior, 44, said that the man, who has been identified as Dzhokhar Tsarnaev, 19, appeared to be "very nervous."
"He was biting his fingernails, and was shaky," Junior said.
Tsarnaev had dropped off the car, which Junior described as a white Mercedes wagon, at the auto shop about two weeks earlier. It had rear bumper damage, and Tsarnaev had said it was his girlfriend's. On Tuesday, when Tsarnaev suddenly returned, Junior told him the car wasn't ready.
"I don't care, I don't care, I don't care, I need the car right now," Tsarnaev said, according to Junior.
The mechanic said Tsarnaev took the car without its rear bumper on.
Junior said he had known the two brothers for about two years. His shop, Junior's Auto Body, is not far from the Norfolk Street apartment in Cambridge that law enforcement searched on Friday. The brothers and their friends -- between six and eight of them, Junior said -- would bring their cars to the shop for work.
"I talked to them all the time, about cars, and about soccer," Junior said.
Junior said he had seen the pictures of the suspects on Thursday, but he wasn't sure it was the young men he knew until Friday morning. With the FBI searching the apartment so close to his shop, Junior went over and told the FBI what he knew. Asked what he had thought of the brothers, Junior said he thought they were "nice" kids. Then he added: "You know, not nice kids now."