A Hacker known as "Konceptor" pleaded guilty to a single charge of breaking into the computer system at Lawrence Livermore National Laboratory in California, which conducts US nuclear weapons research.
Benjamin Troy Breuninger, 22, who goes by the name Konceptor or Kon, pleaded guilty to a single count of unauthorised access and causing damage to a protected computer network, federal authorities in California said in a statement.
Authorities said Mr Breuninger broke into the network on November 3, 1999 and continued to access the lab"s computers for ten days, stealing budget materials and forcing workers to reconfigure the system.
US prosecutors said the reconfiguration cost the lab more than $US20,000 ($38,110).
Mr Breuninger, a Minnesota resident, faces a maximum penalty of five years in jail plus a $US250,000 fine. He is to be sentenced on April 12.