An Alexandria man who shook his 31/2-month-old son to death, and who previously had permanently disabled another infant son, was convicted of involuntary manslaughter by a Fairfax County jury.

The jury recommended yesterday that he serve 51/2 years in prison.

Eric D. Smith, 27, of Garden Drive had faced a possible sentence of up to 10 years. After outlining Smith's prior convictions, Fairfax Commonwealth's Attorney Robert F. Horan Jr. asked the jury to impose the maximum.

"You didn't know he was a convicted child abuser," Horan said, referring to the guilt phase of the trial, in which Smith did not testify and his prior record was not admissible. "You know it now. He's not done it once; he's done it twice."

Smith was arrested Feb. 25, the day after he called 911 to report that his infant son, Dominic X. Smith, was suddenly unconscious in the baby's mother's apartment on Moray Lane in Baileys Crossroads. Dominic was rushed to Inova Fairfax Hospital, where a CT scan showed that he had hemorrhages on both sides of the brain and other significant brain damage, radiologist Alex Lee testified.

Doctors and investigators both suspected shaken baby syndrome, in which violent shaking of a small child causes serious injury or death. Dominic was pronounced brain dead Feb. 26, and after some of his organs were harvested for transplant, an autopsy was performed Feb. 28.

Kathryn H. Haden, an assistant medical examiner, said she found not only brain hemorrhages but a bruise on top of Dominic's head. "In my opinion," Haden testified, "the injuries . . . are consistent with injuries due to both blunt force and shaking." She said they were not likely to have been inflicted in a fall.

Even before the autopsy, Detective Steve Shillingford obtained an arrest warrant charging child abuse and took Smith to police headquarters. He said Smith told him that Dominic's mother had awakened him about 9 a.m. to take care of Dominic while she left the apartment.

Smith told Shillingford that Dominic began to cry, so Smith tried to comfort the boy by rocking him and then placing him in a swing. "At that point," Shillingford said Smith told him, "the child began to go out," meaning his eyes rolled back and he became unconscious. Smith reported splashing water on Dominic's face, then calling 911.

Shillingford said he told Smith he didn't believe the story. Eventually, the detective said, Smith started crying and fell to the floor. "He said, 'You know I don't want to say it,' " Shillingford testified. He said Smith told him, " 'I did it. I did it. I caused it. I abused him.' " He said Smith denied shaking the baby, then didn't want to talk anymore.

Smith's attorney, James C. Clark, explained that his client's statement was more general and referred only to the fact that Smith felt responsible because Dominic collapsed while he was watching the boy. He argued that even if Dominic was a victim of shaking, there was no proof that Smith did it.

Shillingford testified that Smith claimed he had blacked out during the episode, and court records indicate that Smith also said he "blacked out during the incident in 1994 as well." When Smith was 17, he was convicted in Alexandria juvenile court of the abuse and neglect of a 6-month-old boy, who Horan said was also Smith's son.

Horan said that boy was left severely retarded and using a wheelchair as a result of a beating inflicted by Smith. But Virginia law allowed Horan to present Smith's conviction to the jury only in the sentencing phase -- and even then he could not discuss details of the case or offer any supporting testimony. Smith served about six months in juvenile detention for the first child abuse conviction, Horan said.

Smith also had convictions for burglary and larceny at age 17, assault on a police officer at 19, cocaine possession at 20, two probation violations resulting in four years of prison time, and a misdemeanor assault on an adult family member last year.

Dominic Smith's mother, Natalie Padilla, said Eric Smith did not deserve his conviction. "He was a good father," she said. "He was there for everything. We're standing by him."