An off-duty St. Louis County police officer fatally shot a man after he broke into the officer's home Saturday evening, police said.

Police said the off-duty officer, who was not identified, shot and killed 20-year-old Tyler Gebhard after he threw a 50-pound concrete planter through a window to enter the Lakeshire home, the St. Louis Post-Dispatch reported.

According to police, Gebhard was known to the family and had recently made online threats toward it and "uninvolved members of the community."

An uncle of Gebhard, Patrick Brogan, 57, of Waterloo, Illinois, told the Post-Dispatch that Gebhard and the officer had been arguing on Facebook about Black Lives Matter. He said Gebhard, who was biracial, empathized with the group's objectives but did not participate in protests, the Associated Press reports.

Brogan said Gebhard and the officer became acquainted through a church connection. He said his nephew, a former Affton High School football standout who graduated in 2014, suffered from bi-polar disorder and sometimes didn't take his medicine.

"I don't think the officer had a choice — I honestly don't," St. Louis County Police Chief John Belmar said.