The suspect was apparently connected with the Chechen jihadists. From MosNews, with thanks to Anthony:
Moscow police intercepted a car packed with explosives in central Moscow on Saturday and said they had thwarted a terrorist attack, Russian media reported.
Police found a 200-gram block of TNT, two anti-personnel landmines and a 20-liter canister with gasoline along with detonators and an electronic operating device in an old Lada.
A suspect arrested at the scene told investigators he was offered $1,000 to take the car to a major thoroughfare President Vladimir Putin uses to travel to and from the Kremlin, news agencies and television said, quoting security forces. The Kommersant daily has cited an unnamed source in the law enforcement agencies as saying that the suspect, whose name was reported as Pumane was a Russian naval officer on a submarine.
The daily also reported that the car in which the explosives were found was registered in the name of a woman who supposedly was a wife of a Chechen rebel.
Pumane died in hospital six hours after questioning. Initial reports said the man had suffered a heart attack, but shortly afterwards it was reported that he had bean beaten to death, apparently during the interrogation.