US line Matson Navigation is investigating how 12 empty containers were lost from the deck of a boxship off California.

The 3,027-teu Manoa (built 1982) was heading to Seattle from Oakland in rough seas on Friday when the accident occurred, the San Jose Mercury News reported.

The ship was disembarking a local pilot and took a swell side-on, Matson spokesman Keoni Wagner told the newspaper.

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