Visa shares dropped sharply Friday after a report said the Justice Department has opened an investigation into its debit card business and possible anticompetitive practices. The company said Friday afternoon that investigators have started to collect information on Visa debit practices. The Wall Street Journal, which first reported the probe, said the DOJ is seeking to determine whether the largest card network in the United States has curbed merchants’ capacity to route debit-card transactions over less-expensive networks. Sources told the Journal that investigators’ questions are cantered on online debit-card transactions but have also included inquiries about in-store issues. The Justice Department declined to comment.