Crews searched Saturday for four people missing after an overnight boat crash on the Mississippi River in Iowa, although one official said it was mostly a recovery effort.

Iowa Department of Natural Resources spokesman Kevin Baskins said the timing of the collision, about 1:45 a.m. Saturday, made it difficult for rescuers to find people in the water. So, he said, the effort is "leaning more toward recovery" than rescue.

"Officers in boats, at this point, looking for anything we can find," Baskins said, describing the search that was expected to continue at least until dark.

"Once it gets dark, it gets problematic," he said.

Two jon boats _flat-bottomed boats often made of aluminum _ crashed in the O'Connell Slough area of the river, near Burlington, according to the department. One of the boats was carrying 11 people; the other had a single passenger.

Baskins said eight people pulled from the water have been hospitalized. At least one person suffered serious injuries and was flown to an Iowa City hospital, he said. The others were taken to a Burlington hospital.

The names of the missing and injured people have not been released, pending notification of their families.

Des Moines County Sheriff Mike Johnstone told The Hawk Eye that the missing included three men and a woman, all believed to be in their 20s. A message left Saturday by The Associated Press for Johnstone wasn't immediately returned.

Baskins said the crash happened on the main channel of the river and searchers aren't sure whether the missing boaters could have made it to shore.

It's not clear how the crash happened. Baskins said it could take weeks to interview all the victims, collect evidence and reconstruct the crash.

"You don't have skid marks and stuff in boating accidents," he said.

Burlington is in southeast Iowa on the Illinois border, about 200 miles north of St. Louis on the river.