The U.S. attorney’s decision to drop charges against a Temple University physics professor accused of relaying U.S. defense technology information to China is not a typical move, according to former federal prosecutors.

The government indicted Xiaoxing Xi in May, but dropped the wire fraud charges against him Sept. 11. Prosecutors said in their motion to dismiss charges that in the time since the grand jury indicted, “additional information came to the attention of the government” and “that it is in the best interests of justice to dismiss the indictment.”