US Navy documents have linked a Pacific Basin-controlled bulker to an incident in which drones are suspected of collecting data on American warships.

According to documents obtained by military news publication The War Zone through the Freedom of Information Act, the 33,520-dwt bulker Bass Strait (built 2006) was transiting waters off California near several US Navy ships in the early morning hours on 15 July 2019.

The documents, published on Monday, show the Hong Kong-flagged bulker sailed alongside the USS Paul Hamilton, while two other US ships noted upwards of 20 drones operating near the vessels.

“[The USS Paul Hamilton] observed MV Bass Strait likely using [unmanned aerial vehicles] to conduct surveillance on US Naval Forces while transiting to scheduled port of call, Long Beach,” the Navy said in one of the documents.

The drones were described as “quadcopter style” and were able to reach an altitude of approximately 6,400 metres (21,000 feet). They were also said to have continued operating after the Bass Strait left the area.

One of the other two ships, the USS Bunker Hill, reportedly tried to reach the Bass Strait and received no response.

That ship noted 11 drones, while the USS Ralph Johnson counted another 10. That ship said the drones were operating in a safe manner.

Pictures included in the cache of documents show dark spots around the Bass Strait assumed to be drones.

The documents appear to suggest the incident lasted four and a half hours.

Pacific Basin, which has served as both the technical and commercial manager of the Bass Strait since 2013, did not immediately respond to a request for comment after business hours at the company’s Hong Kong headquarters. The ship is owned by single-vessel company Bass Straight Ltd, which is listed at Pacific Basin’s address.

The July 2019 incident is among a series of drone swarm incidents described in documents obtained by The War Zone. But it is the only one linked to a commercial ship.