A teenage mother who was caring for her infant daughter and her brother and a family friend, both 9, was fatally shot yesterday morning inside her Landover Hills home by a unidentified gunman, Prince George's County police said. No arrest had been made by last night.

Police said Tiffany Dionna Jackson, 17, of the 6900 block of Standish Drive, was found by officers about 11:20 a.m. after neighbors called 911 to report a shooting. Jackson was pronounced dead on the scene.

Investigators said witnesses saw a man, described as about 20, run out the front door of the two-story brick home immediately after the shooting. Police said they didn't know whether the suspected gunman fled on foot or by car.

Police said there were no signs that anyone had broken into the home but said they had not determined a motive for the shooting.

"We don't know how [the suspect] gained entry," said Lt. Mike McQuillan, commander of the Prince George's homicide division. "We're still trying to piece it all together."

Investigators and neighbors said there were three other people inside the home: Jackson's 1-year-old daughter, Diamond, Jackson's brother and a 9-year-old neighbor.

Police said the children were uninjured but declined to say whether they witnessed the shooting. Police also would not describe where in the house Jackson was slain or how many times she was shot.

A woman who lives nearby on Standish Drive and who identified herself as the aunt of the child who was visiting the house said the child rushed home after the shooting.

The child "came running back home and said, 'Aunt Lisa, Aunt Lisa, Tiffany's been shot!' " said the woman, who declined to give her last name. "So I called the rescue squad, and pretty soon the police were all over the place."

Police said the other children were being cared for by relatives. Family members who arrived at the house after the shooting declined to speak with reporters. Neighbors said Jackson also lived with her mother and a teenage brother.

Phillip Marshall, a friend of Jackson's, said the Jackson home was a favorite spot for children to play basketball and gather for weekend sleepovers.

"Little kids go up there all the time to play," he said. "Everybody knows everybody around here. We've never had any problems like this."

Marshall said Jackson had graduated from Parkdale High School in June and worked full time at a Taco Bell restaurant to support her daughter and family. "She just worked and took care of her baby," he said. "She wasn't into any kind of crazy stuff."

Jackson's colleagues at the Taco Bell on Annapolis Road in Lanham said she was the first employee to be named "Champion of the Week," an award that management created a few months ago. They said she had worked at the restaurant as a cook and cashier for about seven months, often filling the 6 p.m. to 2 a.m. shift.

"She always had a smile on her face and treated everybody with respect," said Robert Glover, a co-worker. "She said she was working here to save money to go to college and to take care of her girl."

CAPTION: Tiffany Dionna Jackson, 17, was a June graduate of Parkdale High.