A simmering social media feud over Black Lives Matter turned deadly when an off-duty police officer in Missouri shot and killed a young black man who had broken into the cop’s home.

The unidentified officer, a veteran of the St. Louis County Police Department, fired two rounds into the chest of Tyler Gebhard after the 20-year-old smashed through the window of his Lakeshire residence on Saturday, authorities said.

The two men, who met through church, had clashed on Facebook over Gebhard’s support of Black Lives Matter prior to the fatal encounter, the young man’s uncle, Patrick Brogan, told the St. Louis Dispatch. Brogan said his nephew, a former football star at Affton High School, was also struggling with mental illness.

More Bloody Aftermath Of Police Shooting Streamed On Facebook

“He had a lot of mental problems the last few months,” Brogan said. “A lot of difficulties in life.”

Police said that Gephard had used social media to make multiple threats against the officer’s family and other members of the community, a town of about 1,400 people near St. Louis. Vocativ found that in the days leading up to his death, Gephard’s Facebook page contained more than a dozen posts related to the fatal police shootings of Alton Sterling and Philando Castile, as well as the deaths of five cops in Dallas at the hands of a gunman who wanted to kill white officers. Several other posts appeared to be screen shots captured by Gephard that show messages sent to his inbox by people angered over apparent comments he made online.

“Lol, BLM sucks,” reads one of the messages sent to Gephard that he published to his page.

A different one began with, “Just reported you to the FBI…”

Another person, sent hours before the young man was fatally shot, wrote: “Kill yourself if you want to be a white killer.”

According to police, Gebhard took his Facebook beef offline just before 6 p.m. Saturday when he showed up at the unidentified officer’s home. St. Louis County Police Chief Jon Belmar said Gebhard used a 50-pound planter to break through the back window after the officer’s wife refused to let him in through the front door.

The officer’s wife, along with other family members, including a toddler and an infant, hid inside a bedroom as the off-duty cop confronted the intruder, Belmar said. The family members told police they heard the officer yell “get down, get down, get down,” before he shot Gebhard twice. The young man died at a nearby hospital.

“I don’t think the officer had a choice—I honestly don’t,” Belmar told reporters on Saturday. “We are sick for both families because it was senseless. It was a senseless death.”