Electronic records of 26.5 million American military veterans were stolen from the home of a data analyst for the Department of Veterans Affairs, the department’s secretary, Jim Nicholson, said.

The data included the names, social security numbers, and date of birth of living servicemen discharged since 1975 and some of their spouses, Mr. Nicholson told reporters on a conference call from Washington.

Mr. Nicholson said the analyst, whom he declined to identify, was working on a project but was not authorized to take the data home with him. He declined to say where the analyst lived or when the burglary occurred, save to say it occurred this month.

The executive director of the Electronic Privacy Information Center in Washington, Marc Rotenberg, said the theft “may be the largest security breach reported in terms of the total number of records taken.” He added: “If agencies can’t protect this information, they shouldn’t collect it.”

Mr. Nicholson said local police believe the burglary is one of a series recently in the analyst’s neighborhood. “They have no reason to believe this was a burglary targeted at this information,” he said.

The FBI and the VA inspector general also are investigating, he said.

The analyst, a career employee rather than a political appointee, was placed on administrative leave pending the outcome of the investigation, Mr. Nicholson said. He declined to say whether the analyst would be fired.