The Education Department was working to fix a software glitch in its student loan Web site after users complained that they could see other people's personal data.
The department said Wednesday that only a "limited number'' of the program's 6.4 million borrowers were believed to be affected after the problem began Sunday, since not all use the online system. It did not specify how many.
The program involves holders of federal direct student loans, not those who have loans managed through private companies.
The department blamed the data breach on a routine software upgrade, conducted by Dallas-based contractor Affiliated Computers Services Inc., that appeared to mix up data for different borrowers when they accessed the Web site. Since Sunday, four borrowers have complained, a spokeswoman said.
The department said it had disabled the malfunctioning parts of the Web program and will not turn them back on until the problem is fixed.
The software glitch is the latest in a series of data thefts and other breaches crossing nearly a dozen federal agencies in recent months. The biggest was the loss of a laptop and external drive containing personal data for 26.5 million veterans and active-duty troops. That equipment, lost by a Department of Veterans Affairs employee, has since been recovered.
The Boston Globe first reported the Education Department's glitch on Wednesday.