US coast guard investigators have boarded a massive cargo ship as part of the ongoing probe into what caused the rupture of a California oil pipeline that sent crude washing up on beaches.

The Associated Press (AP) reported on Wednesday that the Rotterdam Express made a series of unusual movements while anchored in the closest spot to where the break in the pipeline occurred, according to data collected by a marine navigation service.

The coast guard is investigating whether a ship anchor might have snagged and bent the pipeline owned by Amplify Energy, a Houston-based company that operates three offshore oil platforms south of Los Angeles.

The AP reviewed more than two weeks of data from MarineTraffic, a navigation service that tracks radio signals from transponders that broadcast the locations of ships and large boats every few minutes.

That data shows the Rotterdam Express, a German-flagged ship nearly 1,000ft (305 metres) long, was assigned to anchorage SF-3, the closest to where the pipeline ruptured off Huntington Beach.

The ship made three unusual movements over two days that appear to put it over the pipeline.

In a statement to the AP on Thursday, Hapag-Lloyd, the shipping company that operates the Rotterdam Express, confirmed that investigators boarded the ship while it was docked at the Port of Oakland on Wednesday.

The company has said it played no role in the oil spill.

"We are fully cooperating with the authorities at this moment," said Nils Haupt, a spokesman at Hapag-Lloyd's headquarters in Hamburg, Germany.

A US official told the AP on Wednesday that the Rotterdam Express has become a focus of the spill investigation.

The official cautioned the ship is only one lead being pursued in the investigation, which is in the early stages.

The investigators are seeking to collect tracking and navigational information from the vessel that could help them identify its exact movements, the official said.

They are also seeking preliminary interviews with at least some crew members.

Petty officer Steve Strohmaier, a coast guard spokesperson, declined to comment on the Rotterdam Express on Wednesday but said the agency is analysing electric charting systems from its vessel traffic service to see which ships were anchored or moving over the spill area.