A man fatally stabbed a woman in the neck at a bus stop in downtown Pittsburgh and then stabbed another passerby on Thursday in what police said was a random attack.

The incident began when an officer saw a woman sitting at a bus stop in some medical distress, so he got out to check on her. A man then came up and stabbed her, and then stabbed another woman who happened to be walking down the street, said Pittsburgh Police commander of major crimes Victor Joseph.

The officer on scene saw a knife in the man's hand and ordered him to the ground at gunpoint, Joseph said. The suspect dropped the weapon and was taken into custody.

The first woman was taken to a hospital and died of her wounds, and the second woman is believed to be in stable condition, Joseph said.

Police do not yet know of any motive.

"It seems by all accounts a random act of violence. Thankfully we had an officer on scene and he took appropriate actions," he said.

Affiliate video from WTAE around noon ET on Thursday shows police putting evidence into large brown bags at a bus stop downtown.