PACIFICA — What appeared to be a ship in distress on Saturday morning turned out to be an unusual piece of flotsam — a 40-foot-long metal shipping container that escaped its moorings in high seas and headed Saturday evening toward a landing at Rockaway Beach.

It’s possible, but not likely, that more of the containers that escaped the cargo ship Manoa may be on their way.

About 11 p.m. Friday, amid rough seas, 12 empty containers slid off the deck of the Seattle-bound Manoa, according to its owner, Matson Navigation Co. The ship was maneuvering to disembark a local pilot, who customarily guides arriving and departing ships, and took a swell broadside, Matson spokesman Keoni Wagner said.

In the ensuring rolling, the containers slid into the sea, about 8 miles outside the Golden Gate. The ship had left the Port of Oakland earlier in the day.

According to the U.S. Coast Guard, storm-churned 19-foot swells and 20 mph winds made for rough going, Lt. Jake Urrutia said.

If the container is well-sealed and enters the ocean at the right angle, it could float, Wagner said. It’s likely, he said, that most of the containers sunk.

A Coast Guard search on Saturday spotted two containers at sea, Urrutia said.

“This is an unusual event,” Wagner said. The containers typically are secured either to the ship’s deck or to each other, and Matson is looking into how they broke loose. An inspection determined that all the remaining containers were secure, Wagner said.

Early reports to Pacifica police and the North County Fire Authority at 10 a.m. Saturday included possibly a ship in distress. Local officials later heard from the Coast Guard that it was a container, not a troubled ship, at sea.

The offshore currents are expected to nudge any floating containers toward Pacifica. Matson has contracted with an agent to collect any arriving containers, Wagner said.

As for the rest, Urrutia said, “we are putting out a safety information broadcast to let mariners know that there are containers out there, so they don’t become a navigation issue.”

The Pacifica Police Department asks anyone seeing the shipping containers land on shore to contact police at 650-738-7314.

Contact Sharon Noguchi at 408-271-3775. Follow her at Twitter.com/noguchionk12.