Italian police say two young American tourists have confessed to fatally stabbing an Italian paramilitary policeman investigating the theft of a bag with a cellphone.

In a statement Saturday, Carabinieri officers investigating the death Friday of officer Mario Cerciello Rega, 35, said the Americans, both 19, have been detained for alleged murder and attempted extortion.

Separately, an investigator who declined to be named said the two had snatched the bag of a drug dealer in Rome who had given them "a different substance" instead of cocaine.

The Carabinieri said the Americans demanded a 100-euro ($112) ransom and a gram of cocaine to return the bag. The man called police, saying he had arranged a meeting with the thieves to get his bag back. There was a scuffle at the meeting and the policeman was stabbed to death.