Two employees of the Moscow subway were detained by investigators on Wednesday in the wake of a deadly rush-hour derailment.
The accident inside the tunnel on Tuesday killed 22 people and hurt 136, many with life-threatening injuries. More than 1,100 people were evacuated from the train, which was stuck between two stations in Moscow's west, in a rescue operation that lasted more than 12 hours.
The Investigative Committee said in a statement Wednesday that they have detained the head of the track department and his deputy. Investigators said a new rail switch at the point where the train left the tracks was not properly installed during repair works in May and was in fact attached to the rail by a single, three-millimetre wire.
There is another line under construction nearby and the switch was to direct the trains to a new tunnel once the line is launched.
The two men have been questioned, and the investigators are preparing to file unspecified charges against them, the committee's spokesman Vladimir Markin said.
The Russian capital's airports and transit systems have been a prime target for terrorists over the past two decades, but multiple officials vigorously dismissed terrorism as a possible cause.
In recent months, subway workers have complained of declining safety standards in the system and disorganized measures for aiding stricken passengers.