Mr. Sharkawa , whose alias is Abu Talha , met as often as once a month with Mr. Zarqawi south of Mosul and in western Iraq , and when he was captured was Mr. Zarqawi 's most important lieutenant still operating freely in Iraq , said Col. Robert Brown , the commander of the 1st Brigade of the 25th Infantry Division in Mosul , whose troops were involved in seizing Mr. Sharkawa .