The e-mails resulted in "the loss of control of top-secret restricted data," Reps. John Dingell and Bart Stupak, both Michigan Democrats, wrote Thursday to U.S. Energy Secretary Samuel Bodman.

Several Los Alamos officials in January used open e-mail networks to share classified information on the characteristics of nuclear material used in weapons, Dingell and Stupak wrote.

When the e-mails were discovered, a security team from Lawrence Livermore National Laboratory was sent to recover laptop computers used to send the e-mail, the Albuquerque Journal reported Friday.

Los Alamos officials declined to comment, citing issues of national security.

Dingell and Stupak said Congress was not informed about the incident, despite ongoing investigations into security lapses at Los Alamos, which designs, builds and maintains U.S. nuclear weapons.