WASHINGTON — Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives used a secret, off-the-books bank account to rent a $21,000 suite at a Nascar race, take a trip to Las Vegas and donate money to the school of one of the agents’ children, according to records and interviews.

Agents also used the account to finance undercover operations around the country, despite laws prohibiting government officials from using private money to supplement their budgets, according to current and former government officials and others familiar with the account.

The revelations highlight the lax oversight at the A.T.F. that allowed agents and informants to spend millions while avoiding the normal accounting process. The Justice Department’s inspector general, who is investigating the secret account, criticized the A.T.F. recently for mismanagement and said the agency did not know how many informants it had or how much they were paid.

The New York Times revealed the existence of the bank account in February, prompting an investigation by the House oversight committee. The Justice Department, which oversees the A.T.F., has denied any wrongdoing, and the department has refused to say whether the bureau continues to operate such secret accounts, which the government called “management accounts.”