As the Manhattan district attorney’s office steps up the criminal investigation of Donald J. Trump, it has reached outside its ranks to enlist a prominent former federal prosecutor to help scrutinize financial dealings at the former president’s company, according to several people with knowledge of the matter.The former prosecutor, Mark F. Pomerantz, has deep experience investigating and defending white-collar and organized crime cases, bolstering the team under District Attorney Cyrus R. Vance Jr. that is examining Mr. Trump and his family business, the Trump Organization.
