George LaMonica, a 35-year-old computer consultant, said he bought his two-bedroom condominium in Norwalk, Conn., from Mr. Shahzad for $261,000 in May 2004. A few weeks after he moved in, Mr. LaMonica said, investigators from the national Joint Terrorism Task Force interviewed him, asking for details of the transaction and for information about Mr. Shahzad. It struck Mr. LaMonica as unusual, but he said detectives told him they were simply “checking everything out.