NEW YORK (AP)—Authorities said the failure of a transformer caused a Staten Island ferry to lose power and slam into a pier.

Staten Island ferry Chief Operating Officer James DeSimone said on Thursday that one of the transformers that helps regulate power to the main propulsion engine broke down.

He told the Staten Island Advance newspaper that three other transformers then deactivated. He said it’s unclear why that happened. Fifteen passengers were hurt when the ferry struck the wood pilings lining slip No. 5 at St. George Ferry Terminal on Wednesday night. All the injuries were minor.

The city Department of Transportation and U. S. Coast Guard are investigating the crash. It happened at the same terminal where a ferry crash in 2003 killed 11 people.