COOPER TOWNSHIP, Mich. (AP) -- Authorities were seeking a pickup truck due to reports that it was driving erratically just before it struck a group of bicyclists in  Michigan, killing five.

Four other bicyclists suffered serious injuries in the crash early Tuesday evening in Cooper Township and were being treated at area hospitals, Kalamazoo County Prosecutor Jeffrey Getting said during a news conference Tuesday night.

"Our thoughts and prayers are with the families of the victims," Getting said. "I can't even begin to imagine what they're going through."

The driver of the pickup, described as a 50-year-old western Michigan man, fled from the wrecked vehicle, and police caught him a short time later. He remains in custody. No charges have yet been filed, but they could come by Thursday, Getting said.

Authorities began receiving calls about a blue pickup truck about 30 minutes before the crash, he said.

Concerns were "about the manner in which that vehicle was being driven," Getting added, without addressing possible reasons for the erratic driving.

Several agencies were searching for the pickup when the first report of the bicyclists getting hit came in just after 6:30 p.m., he said. No officers were actively pursuing the truck at the time it crashed.

The bicyclists had been riding as a group. Their names and ages were not immediately released because family members were still being notified. The group included men and women but no children, as an official had said previously.