The Port Authority of New York and New Jersey violated federal securities laws when it borrowed money to repair a bridge and roads in New Jersey that its lawyers thought were beyond its bounds, securities regulators said on Tuesday.

The agency, which operates bridges, tunnels and airports in and around New York City, agreed to pay a $400,000 fine in a settlement with the Securities and Exchange Commission. The commission, which polices sales of stocks and bonds, said the Port Authority had admitted to wrongdoing in the matter, making it the first issuer of municipal bonds to do so.

But John J. Degnan, the chairman of the Port Authority, said the agency had not admitted to willful wrongdoing, only to negligence in its failure to disclose all of the risks faced by buyers of its bonds. “I was shocked when I saw the release,” Mr. Degnan said, referring to the commission’s announcement of the settlement.

A lawyer representing the Port Authority, James Sottile, sent a letter to the commission demanding a retraction of its news release.