WASHINGTON — It was an audacious scheme: an attempted inside job at the office of a federal watchdog agency, where the cops, the authorities said, became the robbers.

Three employees in the inspector general’s office for the Department of Homeland Security stole a computer system that contained sensitive personal information of about 246,000 agency employees, according to three United States officials and a report sent to Congress last week. They planned to modify the office’s proprietary software for managing investigative and disciplinary cases so that they could market and sell it to other inspector general offices across the federal government.

The Homeland Security Department is investigating, along with the United States attorney’s office for the District of Columbia.

The personal information included names, social security numbers and dates of birth — a rich trove of data not unlike those stolen from other government agencies in high-profile cases in recent years. On the home computer of one of the suspects that was seized during a raid in the spring, investigators also found about 159,000 case files.