The killings and robberies are believed to have begun six years ago, RIA Novosti reported Thursday.
A spokesman for the Investigative Committee said a 46-year-old woman, her 25-year-old daughter, a former traffic police officer and his wife have been arrested in connection with the crimes in the Rostov region.
Andrei Larionov, head of the local Interior Ministry, said the suspects allegedly began their spree in 2007.
Home appliances stolen from the home of a drug police officer and his wife killed in a robbery in 2008 were found in a search of the home of traffic police officer Sergei Sinelnik, Investigative Committee spokesman Vladimir Markin said in a statement.
Sinelnik was fired after his arrest.
The 25-year-old suspect, Viktoria Tarverdiyeva, was injured in a shootout with police Sunday in which two police officers and a suspect were killed, the spokesman said.
Searches of the suspects' homes in Stavropol turned up 20 firearms, including automatic rifles, grenades, shotguns and ammunition, Markin said.
Some of the suspects have allegedly confessed to killing a number of people in the Rostov region, including police officers and security guards, investigators said.