Police are asking for help in investigating an incident in which a man attending a Black Lives Matter protest in Austin, Texas, was fatally shot.

Garrett Foster marched in downtown Austin on Saturday to fight against police violence while pushing his fiancée in a wheelchair, reports The Dallas Morning News. Foster and Whitney Mitchell, a quadruple amputee, had been attending protests in the past few weeks. Foster, a white man, and Mitchell, a Black woman, were passionate about racial equality.

Foster, along with the rest of the demonstrators, was marching down 4th Street and Congress Avenue when a driver turned down the road and started honking their horn. Protesters said that the driver “aggressively accelerated” into the crowd, according to KXAN.