The other seven passengers swam to safety or were rescued as the boat sank.
Vladimir Markin, spokesman for the main investigative agency, laid the blame on the owner and captain of the boat, who was among those killed.
Markin said the owner, Gennady Zinger, had been found in violation of regulations three times in the past, including for exceeding the boat’s maximum capacity of 12. The boat was carrying 16 people when it crashed at 1.30am.
He said investigators have questioned the survivors and four crew members of the Oka barge who had helped to rescue them.
The survivors included a man identifying himself as a citizen of Turkey and another who said he works at the US Embassy, Markin said. The citizenship of the embassy employee was not known.
Crews were working to lift the boat, which was lying on the bottom of the river and underneath the barge.
The Moscow River, which runs past the Kremlin as it makes a large loop through the Russian capital, is plied by a steady stream of freight barges and tour boats on summer days. Private motor boats have become more popular in recent years, although their numbers remain small.
Yesterday’s accident occurred three weeks after more than 120 people died when a double-decker boat capsized and sank on the Volga River. Investigators said the 55-year-old vessel, the Bulgaria, was overloaded when it sank in heavy wind and rain. They have not yet determined what caused it to capsize. Survivors reported the boat was listing to starboard and having engine trouble even as it left the dock.
President Dmitry Medvedev then ordered the inspection of all passenger vessels in the country.
After yesterday’s crash, the Moscow city government said it was considering imposing stricter controls on all vessels entering the central section of the Moscow River, according to a river transport official.