Boston.com -- Army Captain Christopher J. Sullivan was a protector, his sister said. The 29-year-old Princeton native always tried to keep his family and friends from worrying, often telling them the situation was not as grim as it appeared in Iraq, where he had been sent one January ago.

Amy Lilley, Sullivan's sister from Scarborough, Maine, said she had heard from her brother on Jan. 10. ''We were passing around joke e-mails," she said.

Lilley was still trying to let the news sink in yesterday that her brother, a committed career soldier, was killed on Tuesday in Baghdad, while serving as part of Operation Iraqi Freedom.

The Department of Defense confirmed yesterday that Sullivan died when an improvised explosive device detonated near his parked vehicle.

He was scheduled to return home to his wife and son in a few weeks, the government reported.

Sullivan had been working as an armor officer, in charge of the tanks and vehicles in the field, said Maureen Ramsey, a public affairs specialist for the Defense Department.

He had entered the Army in March 1998 and just this month had taken on the responsibility of company commander, leaving a job at headquarters.

He also had served in Kosovo and Germany and in November had participated from Fallujah via satellite phone link in the Veterans Day ceremony held in Shirley.

Sullivan felt it was important to thank veterans who had preceded him and always emphasized that soldiers in Iraq were proud to serve, his family said.

Sullivan's interest in military service started early. At age 14, he joined the Civil Air Patrol. He served in the ROTC while attending the University of Massachusetts at Lowell, where he majored in mechanical engineering. He sought to continue the family tradition of military service, following the path of his grandfather, father, and two uncles.

His parents, longtime Princeton residents James and Dorothy Sullivan, could not be reached for comment. Yesterday, they were with Sullivan's widow, Sandy, and his 19-month-old son, David, in Fort Hood, Texas, where he was based.

Sullivan also leaves an older sister, Jennifer Orr, who lives in Epsom, N.H.

Lilley said Sandy Sullivan had talked to her husband on Saturday.

''This is the worst thing that we could imagine," she said.