ROSEDALE, Md. (AP) — Federal investigators planned to spend the coming days piecing together what caused a chemical-carrying CSX train to collide with a garbage truck, derail and explode at a rail crossing in suburban Baltimore.

In the third serious derailment this month, a dozen or so rail cars — at least one carrying hazardous materials — went off the tracks around 2 p.m. Tuesday in Rosedale, Md., a suburb east of Baltimore. Several rail cars caught fire, sending a plume of black and gray smoke into the air that could be seen for miles, and an explosion rattled homes at least a half-mile away.

At the derailment scene Wednesday morning, the mangled truck lay on its side on the side of the railroad tracks, its contents littering the ground. The charred, derailed freight cars were about 200 yards west of the crossing where the locomotive hit the truck.

Crews were using heavy equipment to clear trees, brush and other debris that were in the path of the derailed train. The corrugated metal walls of a warehouse next to the track were bent and warped.

While local officials breathed a sigh of relief that only one person — the truck driver — was seriously injured and that the chemical fire didn’t pose a greater risk to nearby residents, some areas of inquiry for investigators were beginning to take shape.

Robert Sumwalt of the National Transportation Safety Board said late Tuesday that the collision occurred at a private crossing where the only marking was a stop sign. He said it wasn’t clear why the truck was crossing the tracks or whether it was authorized to be there.

The truck driver, 50-year-old John J. Alban Jr., remained in serious condition Wednesday morning at Shock Trauma in Baltimore, a hospital spokeswoman said. Two CSX workers aboard weren’t hurt.

A team of 15 NTSB investigators was on the scene and would likely remain there for up to a week, Sumwalt said.