The men taken in for questioning by Russian authorities are metro workers, a track supervisor and assistant track supervisor. They remain under arrest and charges are expected to be filed against them shortly.
"The work was carried out improperly," the Russia's Federal Investigative Committee said in a statement explaining the arrests and early inquiry results. "This resulted in the train coming off the rails, the deaths of 21 people and various injuries to more than 150 passengers."
Initial reports suggested a power outage was to blame for a derailment. But now human error seems to be to blame. Officials quelled suspicion the accident was the work of militants.
TV cameras in the Russian capital captured bloodied passengers being carried away on stretchers in the wake of the accident. Some were airlifted to area hospitals.
"It braked very hard. The lights went off and there was lots of smoke," one male survivor told a reporter with Rossiya-24, a local television station. "We were trapped and only got out by some miracle. I thought it was the end. Many people were hurt, mostly in the first rail car because the cars ran into each other."