A father and son are accused of intentionally releasing thousands of gallons of oil into two Pennsylvania streams in the Allegheny National Forest, killing an unknown number of fish, reptiles and amphibians.

Police said Andrew Horton dropped off his son Christopher at tanks where the younger man opened valves and damaged equipment, spilling at least 10,000 gallons of oil sometime Saturday or Sunday.

The Department of Environmental Protection originally estimated that about 36,000 gallons had been spilled in the northwestern Pennsylvania forest, but spokeswoman Freda Tarbell said Tuesday that getting an exact figure was difficult because of the damaged equipment.

About 2 miles of Indian Run and about 4 miles of Chappel Fork were polluted by the spill. An in-depth environmental assessment will begin this week, Tarbell said.

“What makes this especially painful, however, is that Chappel Fork is one of the very rare places in Pennsylvania where wild trout naturally reproduce,” Tarbell said.

A small amount of oil made it through containment booms in the streams and caused a “slight sheen” in part of the Kinzua Reservoir downstream, Tarbell said.

According to state police, Christopher Horton, 22, opened valves on about 20 of Snyder Brothers Inc.’s oil tanks. He also smashed eight meter boxes worth about $2,000 each on the tanks and damaged a generator valued at about $500,000, police said.

The Hortons had been employees of Snyder Brothers, a Kittaning oil firm, but the company would not comment on their employment status or a possible motive. “It’s for the state police to determine motive,” spokesman Dan Giovannitti said Tuesday.

David O’Hara, Snyder’s vice president, said in a statement that most of the spill had been contained and that the company was trying to determine how much oil was released. The sites had security measures, but they were overcome, he said.

Christopher Horton was charged with causing a catastrophe, risking a catastrophe and related crimes. Andrew Horton, 42, of Bradford, was charged with conspiracy to cause a catastrophe, liability of the conduct of another and related offenses.

The defendants were being held in the McKean County jail, and it was unclear whether they had attorneys. A telephone message left for the only Andrew Horton listed in Bradford was not immediately returned.