Two Moscow metro workers have been detained by officials investigating yesterday's fatal crash.
Investigators believe safety breaches may have led to the morning rush hour crash, which killed at least 21 people.
More than 150 people were also injured when the three carriages derailed between Park Pobedy and Slavyansky Boulevard metro stations.
Some media reports put the death toll at 22.
Russia's federal investigative committee said it had detained a track supervisor and an assistant track supervisor.
They are suspected of failing to properly monitor work carried out on a track switch mechanism in an underground tunnel.
"The work was carried out improperly," the committee said in a statement.
"This resulted in the train coming off the rails. A set of points was fixed in place with a piece of regular 3-millimetre wire which snapped."
The committee indicated that higher-ranking officials behind the crash could also be arrested, saying it intended to investigate "absolutely everyone involved in this tragedy".
Russian President Vladimir Putin yesterday ordered a criminal investigation into the crash.
The accident is the worst in years on Moscow's overcrowded metro and highlights concerns about Russia's patchy transport safety record.
There was no sign that the incident was a terrorist attack.
City authorities declared today as a day of mourning as calls mounted to urgently improve the marble-clad but overcrowded metro, one of the world's busiest.
The metro, which first opened in 1935 under Josef Stalin, transports around nine million passengers every day.