By Brad Segall, Jenn Bernstein

WARMINSTER, Pa. (CBS) – Bucks County authorities say an 89-year-old woman caught in the crossfire during a shootout in Warminster in February was killed by a police bullet, but the alleged gunman who fired on police is now facing homicide charges. (see related story).

Bucks County District Attorney David Heckler released his office’s investigation findings into the February 19th event, which happened at the Jefferson on the Creek Apartment Complex.

Heckler says Marie Zienkewicz was hit by a bullet from a patrol rifle fired by a Warminster cop who, he says, believed he was acting to end a threat to the alleged shooter’s girlfriend and other officers. He calls it an excusable shooting.

DA Heckler showed pictures of the scene while explaining the findings at a press conference on Thursday. He said police had to take cover behind a split rail fence, as bullets whizzed by their heads. He added that Cairns fired at least 19 rounds from his apartment on the second floor, targeting police.

Cairns had been arguing with his live-in girlfriend, Deborah Silva. As Cairns shot rounds, Silva made a 911 call asking for police help. At one point, the District Attorney says Silva was standing outside a set of apartment doors, while on the phone with dispatchers.

The District Attorney released pieces of the audio, where the dispatcher can be heard trying to get Silva to leave the area.

Silva doesn’t listen.

Heckler says the 89-year-old woman died as a good neighbor and a good Samaritan who was just trying to help a woman who was in distress.

“She opened her door and in a confluence of unpredictable and tragic events she lost her life in the very act of offering help and shelter to a neighbor,” says Heckler.

He called it a tragic mistake because the apartment doors are not in numerical order and because of some confusion they had their sights trained on the wrong door.

Over the police dispatch radio, DA Heckler says police can be heard saying Cairns’ door was the fourth door from the left, when in fact, his was the third. The fourth belonged to Marie Zienkewicz.

The District Attorney says Zienkewicz opened the door, possibly reaching out to help Silva as she stood outside. That’s when DA Heckler says a “tragic mistake” happened, police fired four rounds believing Cairns was reaching out to possibly harm Silva. Marie Zienkewicz was killed instantly by one bullet.

Heckler says Andrew Cairns is now facing a criminal homicide charge.

“Under the law, he is responsible for the deadly consequences of the chain of events which he began,” says Heckler.

Cairns is being held without bail.