Two pilots were killed Monday night when their air tankers collided over a raging Mendocino County wildfire.

The pilots were fighting a "suspicious" 200-acre fire that quickly burned four structures and threatened more than a dozen others, according to the California Department of Forestry and Fire Protection.

"They were crossing paths with each other," witness Jeff Anderson told KRON-TV in San Francisco. "They did hit direct, and it looked like one might have knocked the tail off the other one, but obviously damage was done to both to make them crash."

Investigators from the National Transportation Safety Board and the Federal Aviation Administration were en route to find out what caused the accident, said Karen Terrill, chief CDF spokeswoman.

"They are extraordinarily skilled pilots who have extraordinary courage," Terrill said. "I have talked to many of the pilots over the years, and they say they believe this is more dangerous than combat flying."

The dead pilots' identities were not immediately released.

One arrest has been made for recklessly causing a fire, a felony offense since it destroyed structures, after at least one person allegedly built a fire pit, CDF Lt. Bob Ceriani said. Another person is being questioned about possible involvement.

The aircraft were single-seater Grumman S-2 submarine chasers used during the Korean War and later acquired as military surplus and converted into tankers capable of carrying 800 gallons of fire retardant. Each gallon weighs 9 pounds, totaling 7,200 pounds of retardant per load, Terrill said. The pilots are capable of dropping the retardant in quarters or all at once, called a "salvo." She was unsure what technique the pilots were using when the crash occurred. She added the conditions make flying very dangerous.

"First of all, the conditions are smoky, secondly they can be turbulent, and thirdly they have to take off and land, take off and land several times a day, which are the two most dangerous parts of flying," Terrill said. "There's a rapid descent and dramatic change in weight when they drop (the retardant) and then ascend rapidly as well always dealing with obstructions like trees, power lines and mountains."

There were nine aircraft involved in fighting the blaze when the accident occurred, said Sgt. Ron Welch of the Mendocino County Sheriff's Department. The remaining aircraft were grounded following the collision, which occurred about 7:30 p.m. about eight miles south of Ukiah.

The pilots worked for San Joaquin Helicopters, a Delano, Calif.-based company CDF has contracted with for a number of years, Terrill said.

No injuries on the ground were reported in the Bus fire, which started Monday afternoon, but some residents voluntarily evacuated their homes.

This was the first CDF crash that occurred while pilots were battling a blaze in the air.