Black nylon line, commonly used for fishing, and other items police declined to reveal were found in foliage near the lookout's two entrances.

FBI officials suspect the explosions were a prank, said John Schiman, FBI special agent in charge. "It doesn't seem like it was terrorism-related."

A solo bike police officer assigned to the Kalihi District, which includes Pali Highway, was treated for ringing in his ears after triggering the first explosion at about 6 a.m. at the Kailua-bound gate.

Another beat officer nearing the end of his shift arrived to open both gates - secured every night at 8 p.m. - and triggered the second explosion about 10 minutes later as he drove through the Honolulu-bound gate. He was not injured.

The booby traps were likely set after the lookout closed Tuesday night and before the officer arrived to open the gates yesterday, police said.

Police could not say who may have been the target of the booby traps. Pedestrians hiking to the lookout through the Kailua-bound entrance or driving in from the Honolulu-bound entrance could have set off the trip wire, said Honolulu police bomb investigator Jimmy Kawakami.

The lookout is part of the Nuuanu Pali State Wayside Park managed by the Department of Land and Natural Resources, said spokeswoman Aulani Wilhelm.

No hiking trails are available and cars are prohibited from pulling off the road by no-parking signs placed strategically along the horseshoe shaped access road leading into and out of the lookout.

Land officials declined to speculate about who may be responsible for setting the booby traps.

More than 1 million people visit the lookout each year, Wilhelm said. Between 8 a.m. and 2 p.m. daily, about 500 vehicles, including 40 buses, drive up to the lookout.

The state Department of Transportation installed the gates at both entrances about a year ago at the request of police to help curb vandalism and thefts from cars at the popular visitor spot.

At the Kailua-bound gate yesterday, police found a shorter piece of trip wire strung 3 to 4 feet from the ground from trees on the roadway's right shoulder to the gate. The other side of the gate did not appear to have been booby-trapped.

The solo bike officer, who did not have the combination to the lock on the gate, apparently drove onto the shoulder, entangling his motorcycle in the trip wire and setting off the explosion.

The device was designed simply to explode, sending shock waves that can blind a person or result in hearing loss, Kawakami said. The officer who was not injured described the explosion as similar to a large firecracker going off.

He apparently drove up a few minutes after hearing about the first explosion, opened the gate, then went around to the second gate before he was stopped by the solo bike officer.

Both officers inspected the second gate but found nothing suspicious.

The beat officer unlocked the second gate and was driving through it when he triggered a black nylon string strung between two no-parking signs on opposite sides of the roadway, setting off the second explosion.