Tuesday night’s Metro train derailment on the Blue and Orange lines most likely was caused by human error, according to two people close to the investigation who were not authorized to speak on the record.

According to Metro chief spokesman Dan Stessel, two Metro employees have been placed on paid administrative leave and are undergoing testing for drug and alcohol use. He said the moves were standard procedure for workers involved in “certain types of incidents.” He declined to release their names.

Stessel declined to comment on initial reports of what happened. He said the train is at the New Carrollton rail yard, undergoing “further inspection.”

“We are looking at the switch and the track area where this occurred,” he said. “We’re talking to the train operator and any other employees involved. Once that data is collected, we’ll be able to draw some conclusions as to the cause, but it is still early at this point.”

The train was carrying about 1,000 people at the time, and no injuries were reported, Stessel said.

Commuters crowd train platforms at the Rosslyn Metro station on June 9, 2008, during peak hours. (Dominic Bracco II TWP)

Metro officials could have more details on the cause in days, he said.

Matt Bassett, chairman of the Tri-State Oversight Committee, which monitors safety at Metro, said the investigation would narrow the cause.

“There is still work to do on how human performance, work processes and equipment may or may not have contributed” to the derailment, he said.

Bassett said the committee sent officials to the scene Tuesday night to assist Metro.

Multiple sources close to the investigation, who spoke on the condition of anonymity, offered similar descriptions of what happened based on initial reports.

Shortly after 7 p.m., an operator pulled a six-car train into Rosslyn station, a key transfer point for the Blue and Orange lines. Passengers disembarked. As the train operator was preparing to pull away from the station, she noticed a red signal on the switch. Switches send trains in the proper direction and are generally considered among the sections of track most vulnerable to derailment, sources said.

The operator called Metro’s operations control center to report the red signal. The sources said the operator was apparently told by the control center that there was a switch problem. The train operator was authorized to get out of the train and obtain help from a nearby crew to manually put a clamp in place to hold the switch in the correct position. The action was supposed to allow the train to advance on the Blue Line toward the Franconia-Springfield station.

It is unknown why the switch was not working, the sources said.

Usually, a computer controls switch movements, according to rail experts. If a switch is not working or a clamp is improperly put in place or fails to work, that can cause a derailment, they said.

In Tuesday night’s incident, once the train operator and crew finished, the control center gave the operator permission to slowly proceed out of the station with the train. She did, but within seconds, the front wheels on the first rail car derailed, the sources said.

The Tri-State Oversight Committee expects to provide more information to Metro at a board of directors safety panel meeting May 10, Bassett said. According to Bassett, the committee has “been in touch” with the National Transportation Safety Board and the Federal Transit Administration.

“We’re just monitoring it right now. We haven’t declared that we’re going to investigate,” Nicholas Worrell, a spokesman for NTSB, said Wednesday.

Some oversight watchdogs, board members and train experts criticized Metro for characterizing the accident as a “minor derailment” in its statements to the public Tuesday night. Any derailment is considered a safety matter, they said.

Mort Downey, chairman of the safety and security committee for Metro’s board of directors, said in an e-mail that “a derailment, especially on revenue service track, is a derailment and by definition a bad thing. What’s fortunate here is that the consequences are apparently minor.”

Bassett said the Tri-State Oversight Committee is “glad no one was hurt and the damage was minimal, but we do take it very seriously.”

According to Metro, the last time a train carrying passengers derailed was in February 2010. A six-car Metro train went off the track at Farragut North. Hundreds of passengers were trapped underground for more than an hour, and three people suffered minor injuries.

Metro concluded that the operator failed to stop at a red signal and pulled onto a side track, then tried to return to the main track, triggering safety devices that caused an automatic derailment. The agency fired the train operator, and the NTSB is still investigating.

Metro has been working on switches throughout its system to make them less vulnerable to problems and potential derailments. Based on an NTSB recommendation, Metro has worked to replace switches throughout the system after a 2007 derailment near the Mount Vernon Square station. Of the 80 people on the train at the time, 23 were taken to hospitals. In its report, the NTSB found that part of the cause of the derailment was not having a guardrail on the switches and Metro’s failure to follow earlier warnings to make the change.

Metro officials said the switch outside Rosslyn was a different type not covered by the NTSB recommendation.