The nature of the warning remained undisclosed, but the Coast Guard said in a statement late Tuesday that a dispatcher with the Vessel Traffic Service, which monitors large ship traffic on San Francisco Bay, warned the tanker before it scraped the bridge Monday.
Coast Guard spokesman Dan Dewell said investigators will examine the recorded conversation between the 752-foot Overseas Reymar and a dispatcher with the service, along with a host of other factors.
No further details about the warning will be released until the investigation is complete, he said.
Mariners and others say the Vessel Traffic Service is a form of air traffic control with one crucial distinction: Its communications are advisory rather than mandatory like air traffic control.
So "warnings" from the service are often phrased as questions rather than direct statements of danger.
"They are not there to order captains around," said Capt. John Konrad, a veteran operator of large ships who now operates the respected mariners website gCaptain.com. "They'll ask a lot of questions."
Konrad and others said expansion of the authority of the service was debated after the cargo ship Cosco Busan unleashed a massive oil spill when it crashed into the Bay Bridge in 2007. But little change resulted because the service only monitors large ships.
"Mariners like me oppose giving them that much authority," Konrad said. "Unlike air traffic control, VTS doesn't know where every small boat on the bay is. They may say turn right not knowing there's a sailboat there."
Jeff Bornstein, a lawyer who represented Capt. John Cota, the pilot of the Cosco Busan, said the service asked "what are your intentions" as the ship steamed for the center of a tower. Bornstein said if the service had issued an explicit warning, Cota may have had time to change course and pass under the bridge safely.
Instead, Cota said he planned to maintain course without opposition from the service, realizing too late that he was misreading onboard instruments.
Still, Konrad, Bornstein and others warned that no conclusions can be made until the Coast Guard releases the recorded communications with the Overseas Reymar.
The Coast Guard has classified the crash as a "major marine casualty" because property damage exceeded $500,000.
Dozens of feet of a protective fender wrapped around the support tower were damaged, but Caltrans said no structural damage was done to the bridge.
In addition, no oil leaks were reported and the bridge remained open. No crew members were injured.
The ship was being piloted by San Francisco bar pilot Guy Kleess, who has been a bar pilot since 2005 after captaining oil tankers and other large ships for companies.
Bar pilots are required by state law to guide every large vessel in the San Francisco Bay and other Northern California waterways.
Kleess lost his pilot license between Nov. 9, 2010, and Jan. 11, 2011, after going on medical leave, state Board of Pilot Commissioners records show.
Charlie Goddyear, a spokesman for the bar pilots association, declined to divulge the details of the medical leave.
The phone at Kleess' home rang unanswered. His attorney Rex Clack didn't return phone calls or email inquiries.
Records also indicated Kleess was involved in three previous accidents. He was held responsible for two and ordered to undergo more training after a ship he was piloting damaged a dock in Stockton in 2009, according to board records.
The medical fitness of pilots became an issue after Cota was found at fault for ramming the Cosco Busan into the Bay Bridge.
Federal investigators concluded that Cota withheld vital medical information from regulators, and that one of the factors in that crash was Cota's use of prescription medication.