Former Republican aide Trevor Blackann pleaded guilty Thursday to filing a false tax return for not reporting more than $4,100 in gifts from lobbyists, and he will assist federal prosecutors in the corruption investigation of former lobbyist Jack Abramoff and his associates.

Blackann, a one-time aide to Sen. Kit Bond (R-Mo.), admitted to accepting gifts between 2000 and 2004, including tickets to the World Series and other sporting events and concerts, as well as travel, meals and beverages.

Blackann attorney Carol Elder Bruce said Thursday that a sentencing date has yet to be set in the case, adding that the ex-Republican aide is working with the Justice Department.

“He is cooperating with the government in its continuing investigation,” Bruce said.

Filing a false tax record is a felony offense that carries a fine of up to $100,000 and a maximum prison term of three years, but federal sentencing guidelines recommend only up to six months in jail, and prosecutors could potentially seek a lesser punishment.

According to documents provided by the Justice Department, Blackann received the majority of gifts — valued at more than $3,100 — in 2003 from an individual identified only as “Lobbyist D,” a close Abramoff associate.

But details provided about “Lobbyist D” match the career trajectory of Todd Boulanger, a former aide to then-Sen. Bob Smith (R-N.H.), who later worked for the firm Preston Gates and then Greenberg Traurig.

Boulanger, now a senior vice president at Cassidy & Associates, did not return a telephone call Thursday seeking comment.

The Justice Department also cites gifts to Blackann from “Lobbyist E” and “Lobbyist F,” including a 2003 trip to the World Series in New York City valued at more than $1,000 counting airfare, game tickets, a souvenir jersey and admission to a “gentlemen’s club.”

Although unnamed, details about “Lobbyist E” match those of James Hirni, a former Senate aide who at one time worked for Greenberg Traurig with both “Lobbyist D” and Abramoff. Hirni, now an executive director of Republican outreach for Wal-Mart Stores Inc., did not return a telephone call Thursday afternoon.

“Lobbyist F” also is not identified, but is listed as an “in-house lobbyist for a construction equipment rental company” that also employed the other two lobbyists in 2003.

According to a 2003 Roll Call report, United Rentals, the nation’s largest tool-rental company, hired both Boulanger and Hirni, to lobby on the company’s behalf.

In addition to lobbying Blackann for issues important to the equipment rental company, the Justice Department outlined other favors the lobbyists sought from Blackann, including a February 2001 request from “Lobbyist D” for a letter of support from Bond for an individual seeking a political appointment in the Bureau of Indian Affairs.

Bond’s office said Thursday that the Senator is not aware of any such letter, and noted that the request actually predates Blackann’s hiring in June 2001.

“Those in government service have a high obligation to observe the law, this former staffer has accepted responsibility and must pay the consequences,” Bond said in a statement. “I have cooperated fully with the investigation and the Department of Justice has indicated that this matter is limited to this former staffer.”

In addition, a Sept. 26 memorandum from the Office of Senate Legal Counsel, a copy of which was provided by Bond’s office, indicates the investigation does not extend to the Senator himself.

“The Justice Department has advised that its request arises from the belief that Senator Bond himself was an innocent victim on potentially improper conduct by lobbyists and former staff,” the memo states.

Blackann, now a lobbyist for the Dairy Farmers of America, is the 15th individual named by the Justice Department in the long-running investigation surrounding Abramoff, and each of those named has either pleaded guilty or is currently awaiting trial.

Blackann, who worked for Rep. Roy Blunt (R-Mo.) before moving to the Senate, is married to Laura Blackann, a one-time spokeswoman to Rep. John Doolittle (R-Calif.), who has also been haunted by his relationship with Abramoff and will retire at the close of the 110th Congress.

FBI agents raided Doolittle’s Virginia home in April 2007, and he and his aides have testified before a grand jury. One former top aide who joined Abramoff’s lobbying team was indicted in September for providing gifts to Members of Congress in exchange for actions they took to benefit Abramoff clients.

Paul Singer contributed to this report.