An off-duty police officer shot and killed a young black man after he broke into his St Louis home Saturday evening, following an argument over Black Lives Matter.

The family, who were in the house at the time, said the victim was known to them and had recently had an argument with the officer about the growing unrest over police violence, according to CBS.

Tyler Gebhard, 20, had rung the doorbell of the Lakeshire home around 5.30pm Saturday, police say. The officer's wife and infant answered the door and after seeing who it was, immediately ran upstairs to hide with her mother-in-law.

An off-duty police officer shot and killed a young black man, Tyler Gebhard, 20, (left and right) after he broke into his St Louis home Saturday evening, following an argument over Black Lives Matter

After trying to enter the house via the front door, Gedhard (right) went round to the back of the house and threw a concrete planter through the door and entered the home (left, the broken door)

Gedhard then went round to the back of the house and threw a concrete planter through the door and entered the home, say investigators.

One inside the home, the off-duty officer allegedly shouted 'Get down! Get down! Get down!' at Gebhard before shooting him twice in the chest, reported St Louis Post Dispatch. As the shots were being fired the family had apparently tried to escape the house through the bedroom window.

The 20-year-old was pronounced dead at St. Anthony's Hospital.

Gebhard had met the family at church and had recently made threats towards them and 'uninvolved members of the community' via Facebook, police told St Louis Post Dispatch Saturday night.

The 20-year-old's uncle, Patrick Brogan, 57, of Waterloo, told the site that his nephew was a former Affton High School football standout and had suffered from bipolar disorder.

His mental health had worsened over the last few months, according to his uncle, and he had 'a lot of difficulties in life. He had to frequently be reminded to take his medication.

Gebhard and the officer had apparently been arguing on Facebook about the Black Lives Matter group. His uncle said that Gebhard, who was mixed-race, had empathized with the group's objectives but didn't participate in protests.

Gebhard had met the family at church and had recently made threats towards them and 'uninvolved members of the community' via Facebook, police told St Louis Post Dispatch Saturday night. Pictured: Officers at the scene

St. Louis County Police Chief Jon Belmar said at a Saturday press conference: 'This is a tragedy for everyone involved'

Hours before he was fatally shot, Gebhard had posted a series of comments on his Facebook page, relating to the racial tension being felt across America.

At 3.20pm he posted a photo of black and white hands clasped together with the words: 'F*** being racist. My white friends are my n***as 2.'

He also posted a photo of a black and white child hugging with the words: 'Children will play with everyone. Until a parent tells them not to.'

Gebhard posted a social media tribute earlier in the day that said: 'Light up Facebook with this blue candle for all our fallen officers.'

Family friend Nikki Toal told the Dispatch that Gebhard was a 'wonderful kid' and 'a sweetheart'.

Uncle Brogan also told the site that he was 'the nicest kid you could ever meet' and was 'just a normal 20-year-old.'

His nephew was dedicated to football and also excelled in golf. He was also the recipient of the Howard Foundation of St Louis Scholarship, awarded by the United Negro College Fund, according to the Affton School District website.

St. Louis County Police Chief Jon Belmar said at a Saturday press conference: 'This is a tragedy for everyone involved.

Hours before he was fatally shot, Gebhard had posted a series of comments on his Facebook page, relating to the racial tension being felt across America (left and right)

Gebhard also posted a social media tribute that said: 'Light up Facebook with this blue candle for all our fallen officers'.

'It's a tragedy for the officer and his family, with two young children in the house, his wife in the house, his mother-in-law.

'It's a tragedy for the suspect's family, regardless of his motives or what kind of emotional distress he was going through - his family has lost a loved one and he has lost his life.'

Belmar added that he didn't think the officer had a choice and said it was a 'very difficult position to be in'.

It is unclear whether the gun he used was the officer's department issued firearm or a personal weapon.

Tributes to the 20-year-old have already begun pouring in on Facebook, with many sharing their memories of the football star (pictured front, second from left, playing baseball)

He said he was 'sick for both families' because it was a 'senseless death'.

Tributes to the 20-year-old have already begun pouring in on Facebook, with many sharing their memories of the football star.

Melinda Wessels wrote: 'My heart is hurting. I got a phone call I didn't want to hear. No matter what you were part of my family. I still can't stop thinking about this. You'll be missed.'

While JaCobion Evans said: 'Started with Tyler Gebhard. First friend i made. First friend i made. Best friend til this day! You Will Be Missed !'