At least seven people were killed and 20 others injured after an explosion outside a Moscow subway station that the Russian security service blamed on a female bomber. The explosion caused carnage outside the station and came just a week after 90 were killed in twin plane crashes over Russia that officials blamed on Chechens. An interior ministry official said one version was that an explosive substance went off under the car parked outside the metro station, but the security service appeared only to be looking at the theory of a 'female suicide bomber'. "With a great deal of probability it can be said we are talking about a premeditated explosion carried out by a terrorist female suicide bomber," an interior ministry official told journalists. Moscow prosecutor Anatoly Zuyev said that seven people had died in the blast and 20 were injured. He said prosecutors opened a investigation for terrorism. News agencies said as many as eight could have been killed. **PHOTO CAPTION*** This TV grab image taken from Russian NTV channel shows a burning car and wounded people at the parking place near Rizhskaya subway station in Moscow. (AFP)