WASHINGTON – The Education Department said yesterday it would arrange for free credit monitoring for as many as 21,000 student loan borrowers after their personal data appeared on its Web site.

Terri Shaw, the department’s chief operating officer for federal student aid, said the people involved are holders of federal direct student loans who used the department’s loan Web site – http://www.dlssonline.com – between Sunday and Tuesday.

It is the latest in a string of data thefts and security breaches affecting more than a half-dozen federal agencies in recent months.

Education Department officials blamed the breach on a routine software upgrade, conducted by Dallas-based contractor Affiliated Computers Services Inc., that mixed up data for different borrowers when users accessed the Web site. Since Sunday, 26 borrowers have complained.

“We’re not pleased and we take this incident very seriously,” Shaw said. “We’ve asked ACS to determine how this glitch was missed in the testing process so we can make sure we fill that gap.”

She said the people affected will be contacted by the department by letter and offered free credit monitoring by ACS.

A message left with the company was not immediately returned yesterday.

The Web site program includes names, birthdates, Social Security numbers, addresses, phone numbers and in some cases account information for holders of federal direct student loans. It does not involve those who have loans managed through private companies.