An overloaded motor boat crashed into a docked barge on the Moscow River in pre-dawn darkness, killing nine of the 16 people on board, officials said.
The other seven passengers swam to safety or were rescued as the boat quickly sank.
Vladimir Markin, the spokesman for the main investigative agency, laid the blame on the owner and captain of the boat, who was among those killed.
Mr 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 also 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, Mr Markin said. The citizenship of the embassy employee is not known.
Crews are working to lift the boat, which is 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.
After the accident, 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 quoted on Ekho Moskvy radio.