DOON, Iowa (AP) — Crews are working to clean up a BNSF oil train derailment in Iowa that dumped crude into floodwaters, while officials seek to get a handle on the extent of the spill and its cause.

BNSF spokesman Andy Williams says 33 oil tanker cars derailed Friday just south of Doon, leaking oil into surrounding floodwaters from the swollen Little Rock River. He said Saturday that the cause of the derailment hasn’t been determined. The amount spilled also isn’t yet known.

Williams says crews have been skimming oil from floodwaters. They’re also building a road parallel to the tracks to try to get to the derailed and partially-submerged oil cars.

The train was carrying tar sands oil from Canada to Oklahoma for ConocoPhillips. ConocoPhillips spokesman Daren Beaudo says each tanker can hold more than 25,000 gallons.