Bridge spokeswoman Mary Currie said a heightened state of vigilance had been declared around the span since the US invasion of Iraq began on March 20 although no specific threat against the bridge has been received.

"We understand that we are a potential target," she said. "We don't wait for a threat to come."

US and California security officials believe the Golden Gate ranks high on a list of potential US terrorism targets, and security on it was hugely boosted after the September 11, 2001, attacks in New York and Washington.

In March, captured al-Qaeda official Kahlid Sheikh Mohammed is reported to have told US interrogators in Pakistan that the terror network considered the bridge and two others in the San Francisco area as possible targets.

Scores of cameras and patrolling guards keep constant watch on the bridge, while highway police inspect selected vehicles for bombs, and Coast Guard vessels closely patrol the waters around it, checking ships passing below.

Security agents responsible for protecting the span, opened in 1937, are also believed to have high-technology devices in place that can detect movements around bridge fixtures at night.

The bridge was shut off to bicyclists and pedestrians in the two months following the 9/11 attacks, and was constantly patrolled by air force jets.

Since then, it has remained open to all traffic and visitors. "We want to keep public access open," said Currie. "We craft our security to this."

Spanning the entrance to San Francisco Bay, the 6,450 foot (two kilometer) Golden Gate connects the city with Marin County in the north.

More than 110,000 vehicles cross it each day, and dozens of commercial ships and pleasure boats pass through the dramatically narrow strait below.

Bridge and Coast Guard officials refuse discuss security precautions or what kinds of threats they have prepared against.

But a car-loaded or ship-borne bomb are considered two possible terror scenarios, in addition to suicide airplane crashes similar to the September 11 strikes.

SPACE.WIRE