The key to making a criminal case against former President Donald Trump is getting his company’s chief financial officer to flip, according to a top prosecutor in Robert Mueller’s special counsel investigation.

There are multiple investigations looking into the 45th president, but Andrew Weissmann, a former Justice Department official and FBI general counsel, sees the most promise in the one being run by District Attorney Cyrus Vance Jr., who is looking for possible bank, tax, or insurance fraud.