POCOMOKE CITY, MD., AUG. 17 -- A two-lane bridge over the Pocomoke River collapsed early this morning, causing no known injuries, but drawing top Maryland highway officials to this town to dispel fears that the state's bridges are becoming dangerous.

Two 36-foot spans of the 285-foot-long bridge tumbled into the river about 1 a.m., officials said, just after a car had crossed it. Maryland troopers rushed to the scene at the edge of this Eastern Shore town and closed off traffic within minutes, officials said.

The cause of the mishap was still not known late today, officials said.

The collapse of the 67-year-old bridge came less than two months after authorities closed the Thomas Johnson Memorial Bridge over the Patuxent River in Southern Maryland to repair structural cracks discovered in the bridge's piers. The bridge, carrying heavy volumes of traffic on Maryland Rte. 4 between Calvert and St. Mary's counties, is scheduled to reopen next week.

At the scene here today, highway officials, including Maryland Transportation Secretary Richard Trainor, were quick to say the collapse was highly unusual and maintenance standards for the state's 4,000 bridges make it unlikely to happen elsewhere soon.

"I think Maryland generally has an excellent record," said Trainor. He said 40 bridges collapse every year in the United States. The last one in Maryland occurred in 1975, he said, at the Choptank River in Denton. There were no injuries in that collapse.

The bridge here spans the Pocomoke River, a narrow but deep tidal stream that separates Worcester and Somerset counties. For many years the bridge formed a main link between Maryland and the lower Delmarva peninsula, but with construction of a nearby four-lane bridge on U.S. Rte. 13 in the late 1960s, traffic dropped off and the bridge is now used largely by local residents.

State Highway Administrator Hal Kassoff, who with Trainor met during the day with city and county officials, told reporters that two main spans collapsed and the piers supporting them were "rolled out of position . . . it looks like an underwater problem."

He estimated it would take 10 weeks to repair the bridge, or a year to replace it, if necessary.

He said he did not know what the repair cost would be.

Kassoff said highway records show the bridge superstructure was inspected last month and the underwater pilings and piers in October 1986, and the bridge was deemed safe.

"We're going to go back in and see if we missed anything in those inspections," he said.

The bridge, built with wooden pilings, carries an estimated 1,200 to 1,600 vehicles a day, including trucks and heavy farm machinery. Signs on the bridge limit truck weights to 25 tons.

Some local residents said many trucks seem to exceed that weight.

"I've never seen a town where they let every heavy tractor and chicken truck use a little bridge like that," said James R. Pilgrim Sr., an owner of Nette's Sweet Shop. "It's just one truck after another."

Trainor said overloaded trucks could have contributed to the collapse and investigators are looking into it.

"It's lucky somebody wasn't on it when it fell," said Leon Sparrow, 82, a retired restaurant owner, who said he last drove on the bridge Sunday.

Two underwater divers arrived late today to begin examining the bridge pilings and to sift through debris at the site. They said they found no evidence that any vehicles had fallen into the 25-foot-deep river.