Federal investigators are looking into why the Washington Teachers' Union paid $20,000 to cater a party for the mayor's chief of staff, Kelvin J. Robinson, and are scrutinizing the relationship between the Williams administration and former top union officials, say sources familiar with the inquiry.

The party in summer 2001 was at the Northwest Washington home of Gwendolyn M. Hemphill, then a top union aide and close political ally of Mayor Anthony A. Williams (D), and her husband, Lawrence Hemphill, at the time a member of the mayor's cabinet. The bill to B&B Washington's Caterer Inc., was paid by a union American Express card that allegedly was used routinely as part of a scheme to misappropriate $5 million in dues money, according to the sources, who spoke on the condition of anonymity.

Investigators are attempting to determine whether Williams or Robinson, then newly hired from Florida, requested the party, sources said.

Even if city officials had no role in arranging it, Robinson's attendance at a costly party in his honor might have run afoul of restrictions in the city's personnel code against accepting gifts from entities that do business with the government. Federal law restricts gifts to city government officials, though court rulings have said cases merit prosecution only if a direct relationship between a gift and an official act done in return for that gift can be proved.

Robinson, who replied to inquiries about the party through an e-mail, said he did not request the party, never asked about who paid and had no reason to believe it violated any ethics rules. He added that receptions for him also were hosted by the city's Latino, gay and Muslim communities, and in each case, the gatherings helped him get to know the city's political community and prepare him for his new job.

"If your allegations regarding this event are true and union money was used to support this event, I find that troubling and most regrettable," Robinson wrote in the e-mail. "No one has spoken with me about this activity, but as is always the case, I will fully cooperate with the proper authorities to ensure that the facts are made known."

The mayor's spokesman, Tony Bullock, said Williams had no knowledge that the union paid for the event. He added that if the allegations prove true, the mayor will repay the cost of the event or any other improper expenditure by the union that might have benefited him. "They will be made whole," Bullock said.

The teachers union and its top officials had a close relationship with Williams and his administration. The union broke with most others in 1998 to endorse the mayor's first campaign, and Hemphill became a top political adviser to him and co-chairman of his re-election campaign. She and former union president Barbara A. Bullock, no relation to Tony Bullock, wielded significant clout on hirings and appointments, current and former administration officials say.

Hemphill resigned from the Williams campaign in October, shortly before the union scandal became public. Her husband left the campaign "by mutual agreement" last month, according to Williams, as the scandal heated up and administration officials sought to distance themselves from the former union officials.

Under Williams, the union negotiated a contract raising salaries of its members by 20 percent over three years. Senior administration officials also intervened in negotiations to secure a benefit sought by Bullock and Hemphill that gave teachers free or discounted legal services, costing the city about $1 million a year.

The party in summer 2001 was billed as an event to welcome Robinson to Washington's political community. More than 100 people, including Williams, came to the party, and there was a band, barbecued ribs and an open bar, say those who attended.

The event last month came to the attention of federal prosecutors investigating the union scandal, in which Barbara Bullock, Hemphill and treasurer James O. Baxter II allegedly misappropriated millions of dollars, according to an FBI affidavit and an audit by the union's parent, the American Federation of Teachers. No charges have been filed against those officials. Bullock's chauffeur pleaded guilty to money laundering last week.

Sources said those prosecutors brought the Robinson event to the attention of lawyers within the U.S. attorney's fraud and public corruption section. Those lawyers are conducting an inquiry in the nominating petition scandal that cost Williams a $250,000 fine and a spot on the Democratic primary ballot last year. They have asked about both the party itself and what contacts with the mayor's office might have preceded it, said one source.

City personnel law prohibits employees from accepting "any gifts, gratuity, favor, loan, entertainment or other like thing of value" from any entity with "contractual or other business or financial relations with the D.C. government" or that "has an interest that may be favorably affected by the performance or non-performance of the employee's official responsibilities."

Robinson quoted the code in a memo last December, warning the mayor's office staff against accepting holiday gifts and attending holiday parties provided by entities that have business dealings with city government.

"Employees, therefore, should not attend holiday celebrations sponsored or hosted by contractors, entities seeking to obtain contracts with the District, or entities regulated by the District," Robinson wrote in that memo.

City personnel rules also would have prohibited the party if it had been paid for by the Hemphills, because gifts from subordinates such as Lawrence Hemphill are prohibited, a fact noted by Robinson in the same memo.

Bill Allison, a spokesman for the Center for Public Integrity, a watchdog group, said that Robinson should have inquired about who paid for the party.

"Ignorance isn't a defense," Allison said. "It doesn't work with traffic violations, and it doesn't work for a $20,000 party."

Benjamin F. Wilson, chairman of the city's Board of Elections and Ethics, said the allegations, which he learned about from a reporter, should be investigated. "The facts as described would require that the Office of Campaign Finance look into this further," said Wilson, whose election board oversees the campaign finance office. "These are serious allegations, and they require careful consideration."

The city's Office of Campaign Finance is investigating allegations that the Williams campaign failed to itemize the costs of fundraisers hosted by Curtis Lewis, a lawyer affiliated with the union, for Williams in 1998 and 2002. It has opened an inquiry into whether work done by Gwendolyn Hemphill for the campaign or campaign phone banks provided by the union amounted to illegal, unreported contributions.

In a separate allegation, Hemphill has alleged that Robinson asked her to pay a bill of $1,200 for T-shirts from the 2000 Democratic National Convention. She has said Robinson expected her to use union money; he said that he expected her to pay from campaign funds.

Kelvin J. Robinson, chief of staff for Mayor Anthony A. Williams, was welcomed to Washington in 2001.