The investigation into why a Mexican Navy training ship struck the Brooklyn Bridge on Saturday, shearing the top of its masts, will look into a possible engine failure and the role of a tug boat that assisted it in backing out of its pier, officials said on Monday.

The ship's engine was the key focus for the National Transportation Safety Board, said Brian Young, who is leading the agency's investigation.

The accident occurred when the majestic white training vessel Cuauhtémoc was departing from lower Manhattan's Pier 17 on the East River shortly after sunset on Saturday.

The ship's planned route was southward, away from the Brooklyn Bridge and towards New York Harbor and her next destination in Iceland.

As it backed out of the pier, the ship was accompanied by a tug boat operated by McAllister Towing. As it cleared the dock, the ship pivoted but continued to move swiftly in reverse in the direction of the bridge.

Minutes later, its three 44.8-metre masts hit the underside of the 142-year-old bridge, causing them to partially collapse.

The strike knocked dozens of white-uniformed naval cadets off the ship's crossbeams, where they were standing ceremoniously for her exit from the harbour. They could be seen dangling from their harnesses high above the ship's deck.

Two people aboard the ship - a cadet and a sailor - were killed, and around 20 were injured, two of them critically.

Investigators are asking the public for additional video of the incident and have yet to determine if the Mexican naval vessel had a data recorder. (Reuters)