FRESNO - At Stanford thousands of staff and students had their e-mail passwords stolen in the past few weeks as hackers gained unauthorized access into the university's security system.

Stanford administration officials told reporters on Tuesday the hackers had broken into the system and gained access to 4,500 e-mail. On Tuesday Stanford discovered the breach in the firewall, and took immediate steps to halt unauthorized access, and to tighten security measures.

Stephen Hanson, director of the school's computer security, told reporters he did not know if Chelsea Clinton's e-mail account had been affected.

The hackers, believed to be based outside the U.S., broke into the Stanford computer system on October 11, 1998 with a data-stealing software program called a sniffer which intercepts passwords as users are logging on.

The University has been stepping up installation the security software upgrades that checks for digital break-ins.