When police officers make an arrest, they commonly interrogate the arrestee. Usually they are trying to strengthen the prosecution's case by getting the arrestee to provide some evidence of guilt.
Al-Nusra was quoted as saying that its captives were from a US backed organization known as Division 30. It is the second time in a week that al-Nusra Front has made such a claim.
At least three of those arrested were alleged to have been guards at Auschwitz between 1942 and 1945 when 1.5 million people were killed in what amounted to an industrial system of murder.
Two nurses and a doctor from the same clinic in Cario are among five people of arrested so far, and police say they are still looking for the hospital manager.
Twenty such impostors were arrested that morning, 18 of whom had come from out of town to impersonate young students who felt they could not pass the entrance exams themselves.
The ambush came two days after militiamen loyal to the opposition party Renamo killed four policemen in Muxungue in an attempt to free several people who had been arrested.
The Interior Minister Bernard Cazeneuve said the authorities have received information that one of those arrested might commit violent attacks but also urged caution.