Russian investigators have announced the arrest of two Moscow metro workers for safety breaches after a train derailed, killing 21 people.
The track supervisor and his assistant are suspected of using inadequate wiring during works on the line in May, the Investigative Committee said.
They have been questioned and charges are expected to be brought soon, it said in a statement (in Russian).
A power surge is said to have caused the crash, which injured 162 people. Wednesday has been officially declared a day of mourning in Moscow.
The two men arrested, named as track supervisor Valery Bashkatov and assistant supervisor Yury Gordov, are suspected of failing to properly monitor work carried out on a track switch mechanism in an underground tunnel.
"The switch mechanism was fixed with ordinary 3-mm wire, which snapped," the Investigative Committee said.
The Moscow metro, one of the world's busiest, is a vital transport artery for the city, transporting more than nine million people on weekdays below its traffic-clogged streets.