san francisco blow-gun dart
This is one of the blow-gun darts that struck a pedestrian walking across the Gold Gate Bridge in San Francisco. California Highway Patrol

Two tourists were hit with sharpened blow-gun darts by a mystery assailant as they walked along San Francisco's Golden Gate Bridge.

A California Highway Patrol officer on the bridge on a bicycle was flagged down by one of the victims, a man, who had been hit in the leg. The five-inch-long (13cm) metal dart had pierced his blue jeans and two inches into his flesh.

As paramedics were speaking with him, a woman approached to report she had been hit by an identical blow dart in the knee. Both were treated at the scene and released.

Police are testing the darts, which resemble very long bulletin board tacks, the length of a pen, for any harmful chemicals.

"I've never heard of something like this, police spokesman Andrew Barclay told the Los Angeles Times. "It seems very random, and it doesn't appear that either of the victims was targeted for any reason specifically. Neither of them had any arguments.

"It was definitely one of those situations where you wonder why would somebody do this," he added. "It is really just reckless, careless. There's no purpose to fire those darts without the intention of harming somebody."

Police speculated that both victims were struck by someone passing in a car. Vehicles crossing the bridge pass very close to pedestrians, and both victims were stuck on the same side and at about the same height as they walked north.

Police in southern California, meanwhile, were seeking those responsible for blow-gun attacks on sea birds after three sea gulls and a duck were found with similar metal darts through their necks.

The birds survived but animal control officers haven't yet been able to catch them to help them.