The police said Monday that they had arrested a 10th suspect in the killing of an off-duty British soldier on a London street. The police said a 50-year-old man was arrested east of London on Monday on suspicion of conspiring to murder Lee Rigby, 25, a soldier who was killed in the Woolwich area of London last week when he was run over by a vehicle and stabbed with knives and meat cleavers. No further details about the suspect’s identity were provided. Two main suspects shot by officers at the scene of the killing remained hospitalized.