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(CNN) —  

An argument between a Connecticut teenager and her mother’s boyfriend over his smoking in the house turned deadly after the man shot the 15-year-old and her 16-year-old brother on Tuesday night and then killed himself, according to the Watertown Police Department.

Della Jette and Sterling Jette Jr. were transported to Waterbury Hospital where they both were pronounced dead shortly after arrival, according to a news release from the Watertown Police Department. Their mother, identified by CNN affiliate WTNH as Danielle Jette, was on the scene when the shootings occurred and called 911, police said. She was not injured.

Paul W. Ferguson, 42, was pronounced dead at the scene from an apparent self-inflicted gunshot wound to the head, police said in the news release.

He had moved into the home two weeks before the incident, police said.

The argument began after Della Jette grew upset by Ferguson smoking in the house, Watertown Police detective and spokesman Mark Conway told CNN. Ferguson then retrieved a handgun from his bedroom and shot Sterling Jette – who had attempted to intervene – in the leg, Watertown Chief of Police John C. Gavallas said in a news conference covered by WTNH.

“The mother went downstairs to call 911 when she heard a second gunshot, presumably when he shot the daughter on the deck,” Gavallas said. “He came back in the house and shot the son in the chest.”

Watertown Police were called to the home at around 9:47 p.m. and arrived to find the teenagers suffering from life-threatening injuries. They were transported to an area hospital where they both died, police said.

The teenagers’ deaths were ruled homicides by the Connecticut Office of the Chief Medical Examiner on Wednesday.

Ferguson was a convicted felon and not authorized to possess a firearm, according to police. He was convicted of first-degree unlawful restraint after an arrest on a sexual assault charge in 2007, Conway said.

Officers found several guns in the house, according to CNN affiliate WFSB.

Both teenagers were students at Kaynor Technical High School, according to police.

Jeffrey Wihbey, the superintendent of Connecticut Technical Schools, said there is nothing worse that can happen to a school community, according to a statement obtained by CNN affiliate WTNH.

“Our number one priority right now is doing all we can to support our students, staff and families through this crisis,” Wihbey said in the statement. “Prior to the start of the school day, Kaynor Tech assembled the school’s crisis team. The team will provide grief counseling to students and anyone in the school community who may need it over the coming days.”

Watertown detectives, the Connecticut State Police and the Waterbury States Attorney’s Office are investigating the incident. Police say Jette is cooperating with investigators.

This is not the first tragedy to occur at the Connecticut home, police said.

Police responded to the home three years ago when Danielle Jette’s husband and Della and Sterling’s father, Sterling Jette Sr., fatally shot himself after a minor domestic dispute, Conway said.

“This mother is completely distraught as you can imagine,” Gavallas said during a press conference. “Boyfriend moves in the house two weeks ago for the holiday season. He’s gone and both her children are gone.”