The 24 crew members (21 men and 3 women)[27] were detained for 10 days in total, and were released shortly after the U.S. issued the "letter of the two sorries" to the Chinese. The crew was only partially successful in their destruction of classified material, and some of the material they failed to destroy included cryptographic keys, signals intelligence manuals, and the names of National Security Agency employees.[2] Some of the captured computers contained detailed information for processing PROFORMA communications from North Korea, Russia, Vietnam, China and other countries.[11] The plane also carried information on the emitter parameters for U.S.-allied radar systems worldwide.[11] The fact that the United States could track PLAN submarines via signal transmission was also revealed to China.[11]