Gunmen turned away from a birthday party in the Bronx returned to shoot up the place Sunday, killing a 17-year-old girl and wounding two other teenagers, police and witnesses said.

Fearing violence, partygoers set up a makeshift checkpoint to frisk people for weapons at the front door of the apartment building in Morris Heights.

The tactic backfired when a gang of thugs got enraged at being stopped and returned to the Morris Ave. building minutes later with guns blazing at 2 a.m.

"I got up and looked out the window [and] I saw nothing but kids scattering," said neighbor Jeannette Dawkins, who heard five or six shots. "I saw one girl howling and one girl crying."

Nadairee Walters, who stepped out of the party to try to break up the fight at the front door, took one slug to the chest. The 17-year-old staggered back inside before collapsing on the lobby's tiled floor. She died at St. Barnabas Hospital.

"I saw her on the floor," said Jordan Jefferson, 18, who was also at the party. "She had a hole in her rib [and] her eyes were spinning."

"I didn't know who was shooting or what they were shooting at," Jefferson added.

Another teen ran to Walters and tried to comfort her until paramedics arrived. Walters was conscious and friends pleaded with her to stay awake. "We were like 'Hang on,' and she said, 'I'm trying,'" said the teen, who would not give his name. "She was a peacemaker."

Two other partygoers, aged 18 and 17, were both shot in the leg and listed in stable condition at Lincoln Hospital, police said.

The gunmen fled after the shooting and remain at large. Investigators were reviewing video taken from a surveillance camera in a neighboring building, a police source said.

Neighbors said they weren't surprised by the shootings and said crime is skyrocketing in the building.

"It used to be the best building in the neighborhood," said Linville Mullings, 78, who has lived in the building for 30 years. "Now cops are here every week."