SAN FRANCISCO - A predawn sighting of a man in a wetsuit steering a motorized raft toward the Bay Bridge on Tuesday sparked a massive multi-agency effort to explain a report that raised grave homeland-security concerns.

But after nine hours of searching San Francisco Bay with boats and a pair of helicopters, officials could offer little to explain the sighting.

The search began soon after 3 a.m., when the captain of the Alameda-based tugboat Polaris telephoned the U.S. Coast Guard to say his boat had nearly collided with a 10-foot Zodiac-style raft about a mile south of the Bay Bridge.

The tugboat's captain reported that the raft was skippered by a man in a wetsuit with a snorkel, but the vessel disappeared into the darkness before contact could be made, said U.S. Coast Guard Capt. Neil Buschman, commanding officer for the agency's San Francisco station.

A massive search was launched, Buschman said, in large part due to the heightened terrorist alert the government issued just four days earlier.

A 41-foot rescue boat was sent onto the Bay, as well as a fleet of Coast Guard cutters. Two helicopters hovered overhead in search of the elusive raft.

Crews from the state Department of Transportation were sent below the bridge deck to search for signs of tampering, and similar inspections were performed along the periphery of the Golden Gate Bridge and the San Francisco waterfront.

Buschman said nothing suspicious was found. Nor did anything turn up that could explain what someone would be doing buzzing about the Bay in a rubber raft at 3 a.m.

The tugboat is owned by Seaway Towing Co. of Alameda, and was returning from a job in Crockett when its captain saw the raft whiz past northbound.

"This raft had no lights, which looked fairly suspicious," said Dale Kline, Seaway's general manager.

"The guy (on the raft) is either incredibly stupid or he was trying to hide something. Given the world we live in, post-9/11, we actually have trained our employees to be on the lookout for anything that appears strange."

The CHP has been conducting random truck searches throughout the Bay Area, including inspections of big rigs approaching the Bay Bridge, since the level orange alert was announced last Friday.

Other landmarks have been under heightened security since the threat level was increased, reportedly on information that populated areas might be targeted in a terrorist attack.

Still, Kline was one of several people who said he believed activities other than terrorism could explain the raft's presence in the shadow of the bridge. Kline said he knows of some people who commute to work by boat on the Bay.

"It could be someone out poaching sturgeon or kingfish," said Carrie Wilson, fisheries biologist for the state Department of Fish and Game in Monterey. "It's not that unusual for poaching to be going on out on the Bay.''

Added Kline: "It was definitely suspicious, but it's not like it would stop me from commuting across the bridge."