"She comes from a whole family devoted to public service," Police Commissioner Edward T. Norris said as he wiped away tears at a brief news conference outside Shock Trauma yesterday morning. "It's a sad day for this department."

Sheffield, 35, is the sixth officer to die in the line of duty since Norris joined the force in early 2000. Four of the other officers also died in car crashes.

Norris was joined at the news conference by police union officials, police officers and fire officials, including Fire Chief William J. Goodwin Jr.

"This is a tragic loss for all of us," Goodwin said, noting that Sheffield was married to a city firefighter, Lt. William A. Sheffield. "People talk about heroes. Most days, the thing they do is show up for work. They never know what their next [call] will be."

Police said Sheffield, who was assigned to the Western District, was responding to a call about 11:30 p.m. to help an officer trying to control a dispute on a West Baltimore street corner. Sheffield was heading east on Lafayette Avenue with her patrol car's lights flashing and siren wailing as she approached Carey Street, police said.

Sheffield slowed at the intersection and began to turn right onto Carey Street when her car was struck on the driver's side by an unmarked police car going south on Carey Street. Officers Daniel Meehan and James Lester, who was driving the unmarked Chevrolet Lumina, were rushing to the same call.

Police said they were unsure how fast the cars were going, but officials believe Sheffield entered the intersection while the light was red. Sheffield, who was wearing her seat belt, suffered a severe brain injury and a broken pelvic bone.

"There was a lot of whiplash force," said Dr. Thomas M. Scalea of Shock Trauma.

Sheffield did not have any brain activity when she arrived at the hospital, Scalea said. Doctors operated twice to try to stem internal bleeding.

Sheffield's husband, son Darian and other relatives were at her side when doctors turned off life-support machines, officials said.

Lester, a two-year member of the force, and Meehan, who has been on the force for a year, were treated for minor injuries at Shock Trauma and released.

Yesterday afternoon, glass and debris from the crash covered a sidewalk near the intersection. On the sidewalk, someone had placed a dozen roses.

After news of Sheffield's death spread through the city's police districts, officers began placing black bands on their badges. Flags were flown at half-staff.

"This is something that's becoming all too common," said Gary McLhinney, president of Fraternal Order of Police Lodge 3, referring to the six deaths in the line of duty. "But it's still devastating."

Colleagues and friends said Sheffield was a meticulous and hard-working officer who loved her job. She graduated from Mergenthaler Vocational-Technical High School in 1984 and worked as a clerk and waitress until she joined the force in 1999 at the age of 32, joining three family members already there.

Her brother, Frederick Allen, is an officer in the community relations division, and her sister, Barbara A. Magness, is a lieutenant in the Central District. Sheffield's brother-in-law, Sgt. James F. Magness, is assigned to the department's inspections section.

Officer Robert A. Jones often worked with Sheffield, who he said could react to almost any situation arising in some of the city's roughest neighborhoods.

Jones remembered one night in July last year when he spotted fireworks exploding in an alley. He and Sheffield slipped into the darkness and arrested several youths for setting off the fireworks and selling marijuana.

"She was willing to go down that alley," said Jones, 32. "We ran into a lot of dark alleys together."

Sheffield was also able to quickly resolve most neighborhood arguments and fights between girlfriends, boyfriends, husbands and wives, Jones said.

"She was good at defusing domestic situations - you know how rowdy people can be," he said. "She was pretty good at doing that. She was compassionate and would hear people out."

Colleagues praised her professionalism and commitment.

"She was aggressive," said Lt. John J. Paradise. "She never complained. She liked to go after drugs. She was always, always, always reliable. She's hard to describe. She took the job seriously. She was a professional. She's going to be missed in this district."

Sun staff writer Josh Mitchell contributed to this article.