Markelov, the lawyer for the family of a Chechen woman murdered in 2000 by Russian army colonel Yury Budanov, was shot dead at around 2 p.m. Moscow time (11:00 GMT) near the Kropotkinskaya metro station in downtown Moscow. Novaya Gazeta journalist Anastasia Baburova, 25, who was with Markelov, was wounded and died in hospital later on Monday.
"According to investigators, this was a contract killing related to the lawyer's professional work. The woman who was with him, a journalist student, became an accidental witness and victim in the crime," the source said.
Witness statements and CCTV recordings have provided police with a full picture of the crime, the source added.
Markelov was killed shortly after giving a news conference on the controversial parole of Budanov and his subsequent release earlier this month. The gun he was shot with was fitted with a silencer giving weight to the contract killing theory.
"An investigation into the lawyer's cases and his most recent telephone conversations is being conducted, and witnesses are being interviewed. Reporters who attended Markelov's press conference an hour before he was killed may also be potential witnesses," the source said.
Russian Prosecutor General Yury Chaika is personally supervising the investigation into Markelov's killing.
