The train that struck another train stopped outside the Farragut West Metro station in D.C. early Monday was under a “zero-speed command,” according to preliminary findings of the investigation.

The two out-of-service trains were returning to their respective rail yards when they collided just before 1 a.m.

“It’s not unique to Metrorail that a train can move under zero-speed commands,” David Mayer, CEO of the Washington Metrorail Safety Commission,  said Tuesday.

Rules allow trains subject to a zero-speed command to move up to 15 mph if permission is requested and granted, which Mayer said did not happen. The train was moving at about 11 mph at impact.

Mayer said the collision disrupted nearly all the interior furnishings in the first rail car of the striking train. There was significant damage to the exterior, and undercar equipment was broken off.

The trains’ operators suffered minor injuries; one was taken to a hospital and released a few hours later.

No track anomalies were apparent in the area of the collision, and the Automated Track Protection System that prevents trains from running into each other did not appear to be an issue, Mayer said.

It is unclear why the striking train was moving despite the command and without permission to do so.

The continuing investigation will examine factors such as whether the operator was tired, distracted, or working overtime due to special trips for a Washington Nationals game.

“This bucket of human factors, we’ll take a look at all of that. Right now we’re really focused on: Is there anything we can do expeditiously to move forward on solutions for prevention?,” Mayer said.

Mayer said potential solutions might include engineering or train modifications to prevent a train from moving if it is really not supposed to. Such modifications already are being explored for 7000 Series trains, but would now include the 3000 Series trains that were involved in the crash.