Both companies help the Prosecutor's Office investigate the causes of a malfunction at the Chagino electricity substation and subsequent power outage on May 25-26.
Prosecutors have launched a criminal investigation into negligence and power abuse which led to major power cuts.
The Prosecutor's Office summoned UES head Anatoly Chubais for questioning on Thursday.
Chubais was four hours late for questioning and arrived at the Prosecutor's Office at 20.00 Moscow time. He left the office after midnight.
According to Sergei Marchenko, a representative of the Moscow Prosecutor's Office, Chubais was questioned, in particular, about the activities of Unified Energy Systems and its branches.
The May 24 malfunction at the Chagino substation in Moscow caused power cuts in five administrative districts of Moscow, as well as in the Moscow, Kaluga and Tula regions.
The outage paralyzed public transport bringing 43 metro trains to a standstill. Suburban and passenger trains were stopped on 656 routes. All in all, 11,706 buildings, including 8,715 residential blocks, 117 kindergartens, 106 schools, 3 boarding-schools, 31 hospitals, 3 aeration station, 159 telephone exchanges and 3 important water supply stations suffered from the outage.
