Michael Adebolajo, 28, who was shot and injured by police in the incident on May 22, faces questioning over the killing of Lee Rigby and the attempted murder of a police officer, the force said in a statement.
Fellow suspect Michael Adebowale, 22, was also shot at the scene and was discharged from hospital on Tuesday, before being charged the following day over the killing.
Police investigating the soldier's murder also today arrested two men on suspicion of being involved in the supply of illegal firearms, Scotland Yard said.
Searches of two houses are still underway, police said.
They were the eleventh and twelfth people arrested as part of the probe into Rigby's murder, which is being investigated by Scotland Yard's counter-terrorism branch.
Of the other people arrested, two women were released without charge and six men accused of conspiracy to murder are out on bail.