Jan. 26 (UPI) -- At least 18 people were killed and 24 remain missing Monday after an overnight ferry sank off the southwestern Philippines.

The Philippine Coast Guard, which pulled 317 survivors from the water, was continuing to search for the missing passengers and crew from the MV Trisha Kerstin 3, a 620-ton roll-on, roll-off ferry that was on its way to Jolo Island from Zamboanga City on Mindanao Island in the far south of the country.

Coast guard spokesperson Capt. Noemie Cayabyab told GMA News that distress call that the vessel issued a distress call without warning about four-and-a-half hours into its journey at 1:50 a.m. local time, but that sea conditions were rough in the area due to a northeast monsoon.

"There is an ongoing investigation into the cause of the sinking," she added.

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