Egyptian newspaper Al-Masri al-Yawm has quoted Mamduh Ismail as saying he met al-Muhajir, also known as Sharif Hazaa, or Abu Ayub al-Masri, in Tura prison in Cairo, where he has been held for seven years.
"Sharif Hazaa [al-Muhajir] is in Tura prison, and I met him two days ago while I was visiting some of my clients," Ismail, a lawyer known for defending Islamist groups, told the newspaper.
Al-Muhajir is on the "most wanted" list issued by the Iraqi government last week. The US military in Iraq has put a $5million price on his head.
The US army media centre in Iraq said: "We cannot comment on the news that ... al-Masri is in an Egyptian prison and not in Iraq, we have to clarify that from the Egyptian government."
The US military had announced after the death of al-Zarqawi that al-Masri had been appointed the leader of al-Qaeda's organisation in Iraq.
The military said al-Masri was born and brought up in Egypt. He then went to Afghanistan, where he trained in bomb-making before going to Iraq in 2002.