Investigators believe the mother of a 3-month-old girl was the shooter in a double-murder-suicide that left her, her infant daughter and her husband dead last week, a police source said.

The three were officially identified by police Monday as Mary Jo Trokey, 32, Matthew Trokey, 33, and their daughter Taylor Rose Trokey. A spokeswoman for the department declined to release further information in the ongoing investigation.

But a police source said Mary Jo Trokey is believed to be the shooter. Investigators are tracking her purchase of a gun in the days before the shooting late Thursday night or early Friday, and are investigating mental illness as a possible cause.

The three were found dead in their home in the 6200 block of Kinsey Place about 3:45 p.m. Friday. The victims were discovered by a parent of one of the adults, who then called police.

A portion of the St. Raphael Catholic Church bulletin from Christmas weekend 2017 announces the baptism of Taylor Rose Trokey. on Feb. 2, 2018, the 3-month-old girl and her parents, Mary Jo Trokey and Matthew Trokey, were all found dead in their St. Louis home in an apparent double murder-suicide.

Authorities have consistently said they believe it to be a murder-suicide, and said they found a gun at the scene. In a summary of the crime released Monday, police named all three, listing all of them as victims with no indication who may have been the shooter.

The Trokey family's pastor told the Post-Dispatch Saturday that he had seen no sign of trouble.

The Trokeys have been attending St. Raphael the Archangel Parish in the St. Louis Hills neighborhood for about two years, said the Rev. Bob Reiker. A church newsletter in December congratulated the couple on the Dec. 17 baptism of their daughter.

Reiker said he performed the baptism. Reiker described the couple as friendly and said they regularly attended Sunday Mass; he said Mary Jo was active in the St. Vincent de Paul Society.

"It's hard to imagine what happened," Reiker said. "People are baffled by it. It's inexplicable how someone could do this to themselves, let alone their little girl."

Speaking at the scene Friday, police Capt. Eric Larson said all three were found in the same part of the home. He said there was no sign of forced entry and no one else was home at the time of the shootings, which police believe happened overnight.

The scene is just southeast of Francis Park, in the St. Louis Hills neighborhood on the south edge of the city.

Police said counselors were made available to the first-responders who were sent to the home.

A relative of the mother who asked not to be identified said the couple, lifelong residents of south St. Louis and South County, had married several years ago. The father worked at an auto accessory and repair shop.

A funeral Mass for the Trokeys is set for 11 a.m. Wednesday at St. Margaret Mary Alacoque Church, 4900 Ringer Road. Kutis Funeral Home is handling the arrangements.