The North Slope had one of its largest crude oil spills this week.

So far there's no evidence that any oil from the 24,400-gallon spill at BP's Milne Point oil field on Monday morning contaminated the tundra, though it's possible that some saline water may have escaped on the snow, state regulators said.

The spill is the biggest on the Slope since BP's record 201,000-gallon oil spill at Prudhoe Bay in 2006, which resulted in congressional hearings and criminal prosecution of BP. That spill did contaminate the tundra.

This week's spill is the sixth largest crude oil spill on the Slope. Milne Point is a medium-sized North Slope field located northwest of the huge Prudhoe Bay field.

A preliminary investigation showed that the spill -- up to 90 percent oil and the remainder saline water -- resulted from a tank overflow at a gravel pad. An electronic component of the tank's automated flow control system failed and the oil spilled into a containment basin and onto the frozen gravel pad, said BP spokesman Steve Rinehart. An alarm sounded and workers nearby were able to cut off the flow manually within about 20 minutes, Rinehart said.

"We are very interested in finding out what happened to this device," he said.

"We take any incident like this seriously," he said.

Regulators said they are focused on cleanup right now, but after that is over they will also investigate why the device failed.

About 15 people are working on the cleanup and a state Department of Environmental Conservation employee is monitoring it, said Ed Meggert, the department's on-scene spill response coordinator.

The cleanup crew is removing gravel from the pad and testing to see how deep the oil has penetrated the pad.

The oil from Milne Point has a thick consistency -- a little lighter than corn syrup but heavier than motor oil, Meggert said.

The second-biggest North Slope crude oil spill also happened at Milne Point in 1989, when 38,850 gallons were released. Another crude leak at Milne Point occurred in 1985, when a valve failed, spilling 7,350 gallons.

This week's spill did not result in any lost production from Milne Point, which produces about 28,000 barrels of oil per day, Rinehart said.