The Hy-Vee market corporate office in Des Moines, Iowa, was closed for a couple of hours on Tuesday afternoon because an employee opened a letter and found a crystalline substance inside the envelope, according to a company spokesman.WOI-TV.com reports police have not been able to identify the substance, but say it is not harmful. Two and a half hours after a hazardous materials crew responded to the office, tests indicated the powder did not pose any threat. Further tests are planned.According to the article, police told the media that workers were allowed to go home after the preliminary tests, but were urged to wash their hands as a precaution."We\u2019re assuming it\u2019s a prank, but we want to make sure that we do everything correctly," Hy-Vee spokesman Chris Friesleben told WOI-TV. "The letter came through the mail and was placed in an inbox of an employee\u2019s cubicle. Corporate security was notified after the employee opened the letter."WOI-TV reports the substance was described as having the texture of salt or sugar.Hy-Vee has more than 200 retail stores in seven Midwest states. Its $4.6 billion in annual sales ranks it among the top 15 supermarket chains in the nation.Compiled by Paul KersteinKeep checking in at our Security Feed for updated news coverage.Or subscribe via RSS.