ALEXANDER, N.D. (AP) — Officials on Friday were investigating the cause of an explosion and fire at a saltwater disposal site in western North Dakota's McKenzie County.

Thursday's explosion destroyed 13 storage tanks and resulted in a spill of 270 barrels of oil and 2,440 barrels of saltwater, a byproduct of oil and gas production. One barrel holds 42 gallons.

The spill was contained on site, and cleanup continued Friday, McKenzie County Emergency Manager Jerry Samuelson said. He declined to speculate on a cause.

"We've had a couple of these in the past year, three or four, and most of them were lightning strikes," he said. "But we didn't have that type of weather yesterday."

The incident happened at a Mesa Oil Services site in a field about eight miles southwest of Alexander. No water sources were contaminated, Samuelson said.

"Lots of the oil and saltwater obviously burned off," he said. "The good thing is that most of it dissipates."

No one was hurt, though Samuelson said a truck driver suffered an apparent heart attack at the scene. He did not know the man's condition.