An off-duty St. Louis County police officer fatally shot a man as he was trying to enter 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."

St. Louis County Police Chief Jon Belmar said the officer is a three-year veteran of the department and has been placed on administrative leave pending the investigation of the incident. He declined to elaborate on the nature of the threats, citing the ongoing investigation.

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.

"As far as a person (he) was the nicest kid you could ever meet," Brogan said. "He was just a normal 20-year-old."

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.

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

According to police, the officer was home Saturday evening with his wife, mother-in-law, a toddler and an infant when Gebhard rang the doorbell. The officer's wife answered the door with the infant and recognized Gebhard, who played football for Southeast Missouri State.

She shut the door and went to a back bedroom with other family members.

Belmar said that before the officer could get to the front door, Gebhard went to the rear of the house and threw the concrete planter through a window to enter the home. Family members in the bedroom said they heard the officer tell the intruder to "get down, get down, get down," and then heard several shots fired, Belmar said.

Gebhard, who was shot twice in the chest, was pronounced dead at a hospital.

"I don't think the officer had a choice — I honestly don't," Belmar said. He called it "a very difficult position to be in."