The Mexican navy tall ship that struck the Brooklyn Bridge was underway for less than 5 minutes before its masts crashed into the historic span, and radio calls indicating it was in distress went out only 45 seconds before the deadly collision, according to a timeline laid out by U.S. investigators Monday.

With the help of a tugboat, the Cuauhtemoc training vessel backed away from a Manhattan pier filled with cheering people at 8:20 p.m. on Saturday, officials said.

Videos showed the ship moving slowly at first, its rigging filled with white lights and naval cadets balanced high on the ship’s yards — the spars that hold the sails. The tugboat nudged the ship along, keeping it from drifting upstream toward the bridge in the current as it backed up into the East River toward Brooklyn.

But after a few minutes, the ship separated from the tug and picked up speed, still moving in reverse, heading for the bridge.

Four minutes after the ship left the pier, a radio call went out asking for help from any additional tugboats in the area, followed by other requests for assistance, National Transportation Safety Board investigator Brian Young said at a media briefing Monday. Officials did not say whether those radio calls originated from the ship, the tug, or somewhere else.

Forty-five seconds after the first call, the ship, struck the bridge, snapping its three masts. Young said it had reached 6 knots, or nearly 7 mph, at the time of the crash. The Cuauhtemoc kept going, passing beneath the bridge and bumping against a pier before ultimately coming to a stop at around 8:27 p.m., Young said.

Footage of the collision shot by horrified onlookers show the ship moving swiftly backwards and then grinding beneath the 142-year-old bridge as its topmasts snapped off. Multiple cadets in the ship’s crew were aloft, standing on the ship’s yards when the collision happened. Several were left dangling by safety harnesses as the masts partially collapsed. Two cadets died.

The tugboat that had helped the ship get out of its berth could be seen on video trying to get ahead of the vessel as it headed toward the bridge but couldn’t overtake it in time.

It remains unclear whether a mechanical problem played a role. NTSB officials said they have not yet been granted permission to board the ship and they have not yet interviewed the captain or the tugboat and harbor pilots who were assisting the vessel as it tried to depart New York for a trip to Iceland.