A young Egyptian acquitted of federal explosives charges was unexpectedly arrested by immigration officials in Tampa. The man, Youssef Samir Megahed, 23, who was born in Egypt, was taken into custody by Immigration and Customs Enforcement agents as he left a Wal-Mart store with his father, according to his lawyer, Adam Allen. He is being held on a warrant signed by an immigration judge. Mr. Allen said the government was trying to deport Mr. Megahed even though federal prosecutors failed to secure a conviction at trial. In a statement, an immigration department spokesman said Mr. Megahed had “been placed into removal proceedings” and would be held until a judge heard his case. He declined to comment further. Mr. Megahed, a former student at the University of South Florida, is a legal permanent resident who has lived with his family in the United States since he was 11.