A man set off on a shooting spree killing an anti-abortion protester outside a high school because he didn't like the activist holding a sign with graphic images of a foetus in front of students.

The gunman drove next to a gravel pit business and shot and killed the owner, who apparently also upset him, police said.

Authorities believe they stopped a third killing by catching up with the gunman before he could kill again.

"The defendant had ill will toward these three individuals - not for the same reason necessarily, but had a grudge," said Shiawassee County, Michigan, Prosecutor Randy Colbry.

Police charged Harlan James Drake, 33, with first-degree murder.

The shootings started around 7.20am across the street at Owosso High School, as parents dropped off students before class.

James Pouillon, a well-known activist in the town, was standing across the street with a sign that pictured a chubby-cheeked baby with the word "Life" on one side and an image of an aborted fetus with the word "abortion" on the other.

Mr Pouillon, known as "the sign man" for his years of in-your-face protests against abortion, was a polarising figure in Owosso.

"His dedication to his cause was unprecedented," said Tony Young, who tangled with Mr Pouillon during protests outside his car dealership.

The county's chief assistant prosecutor, Sara Edwards, said there didn't appear to be a "triggering event" but Mr Pouillon's presence outside the school seemed to aggravate Drake.

It was "the fact that he was outside the high school with his signs in front of children going to school," she said.

Drake then drove seven miles and down a dead-end country road to Fuoss Gravel Co and killed Mike Fuoss, 61, who owned the gravel business, said Shiawassee County Sheriff George Braidwood.

The shootings come a little more than three months after the killing of late-term abortion provider George Tiller at his Kansas church. A man with a long history of anti-abortion views, Scott Roeder, has pleaded not guilty to the murder.

Belfast Telegraph