Authorities were seeking a pickup truck due to reports that it was driving erratically minutes before it struck a group of bicyclists on a street in western Michigan, killing five, a local prosecutor said.

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 officials 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 was previously reported.

Early information is that they were northbound and on the right shoulder of the road when they were struck from behind, Getting said.

Markus Eberhard was leaving a nearby park after fishing and said he nearly was hit by the pickup. He was unable to warn the bicyclists.

"I saw a bunch of bikes hit the front of his truck and a couple of them flew," Eberhard told WOOD-TV.

Paramedics and fire crews rushed to the scene, Getting said.

Few other details were released Tuesday night.

"There's very little I can and will tell you about exactly how this incident happened," Getting said.

He declined to answer several questions about the suspect and said the man's name would not be released until his office has had an opportunity to make a determination about charges.

Cooper Township is south of Grand Rapids and just miles from Kalamazoo. The area still is dealing with the random fatal shootings of six people and wounding of two others in February. Jason Dalton, 45, had been picking up riders for Uber at the time of the shootings. He is charged with murder and attempted murder. His attorney told a judge Monday that Dalton is expected to plead insanity in his defense.