PORTLAND – American Eagle airline officials are not certain why an engine in a Bangor-bound jet failed Sunday night, but mechanics bringing a replacement engine to the Portland International Jetport hope to determine the cause.

“This is not something that happens every day, nor do we want it to,” said Lisa Bailey, an American Eagle spokeswoman. “That’s why we want to take a long hard look at this engine.”

A scheduled 65-minute flight from Boston to Bangor turned into an all-night voyage Sunday for passengers aboard American Eagle Flight 4645.

Their first aircraft turned back to the gate because of mechanical problems. Passengers then boarded a second plane, leaving Boston about 90 minutes behind schedule at 11:09 p.m.

The twin-engine Embraer 135 regional jet landed in Portland less than 30 minutes later, diverted there after the plane’s right engine failed.

“Aircraft will fly well on one engine,” Portland transportation director Jeff Monroe said. “The passengers were not in danger at any time.”

There were 38 passengers aboard the plane. None were injured in the landing, Bailey said.

American Eagle paid for passengers to stay overnight in a hotel before they continued to Bangor by bus Monday morning.