The Senate moved yesterday toward asking the Justice Department for a criminal investigation of a $10 million legislative earmark whose provisions were mysteriously altered after Congress gave final approval to a huge 2005 highway funding bill.

In what may become the first formal request from Congress for a criminal inquiry into one of its own special projects, top Senate Democrats and Republicans have endorsed taking action in connection with the earmark that Rep. Don Young (R-Alaska), former chairman of the House Transportation and Infrastructure Committee, inserted into the legislation.

"It's very possible people ought to go to jail," said Sen. Barbara Boxer (D-Calif.), chairman of the Environment and Public Works Committee, which oversees highway funding.

Young's staff acknowledged yesterday that aides "corrected" the earmark just before it went to the White House for President Bush's signature, specifying that the money would go to a proposed highway interchange project on Interstate 75 near Naples, Fla. Young says the project was entirely worthy of an earmark and he welcomes any inquiry, a spokeswoman said.

"Congressman Young has always supported and welcomed an open earmark process. If Congress decides to take up the matter of this particular project, there will be no objection from Mr. Young," said Meredith Kenny, his spokeswoman. Young also sponsored a $223 million measure to build the fabled "Bridge to Nowhere" in Alaska, a project that was killed in 2005 after it sparked widespread outrage.

Young's critics suggest that the motive for the I-75 provision was campaign contributions from real estate developers who own 4,000 acres of land near the proposed interchange. In February 2005, developer Daniel Aronoff hosted Young and Rep. Connie Mack (R-Fla.) at a highway safety event at Florida Gulf Coast University, followed by a fundraiser that brought in about $40,000 for Young's campaign.

The developers have been trying for several years to build on the land, whose value would increase if there were a nearby interchange for I-75, which runs east-west between the Naples area and Fort Lauderdale.

Reports about the Aronoff fundraiser for Young in the Naples News prompted inquiries from a local FBI office in 2006.

Local planning officials, who never requested money for the interchange, were outraged to learn after the highway bill became law that they were required to spend $10 million on a project they did not want. The Lee County Metropolitan Planning Organization, the recipient of the money, has rejected it three times in the past year.

Earmarks are requests to fund special projects, usually in a lawmaker's home state or congressional district. They often are used for libraries, sewers and other infrastructure, and every five years a new highway bill brings with it billions of dollars in new earmarks.

The total number and dollar value of earmarks rose sharply in the final six years of Republican rule on Capitol Hill, becoming increasingly controversial as law enforcement authorities pursued several corruption cases centered on their use. Even as their numbers shrank last year, congressional earmarks accounted for $18 billion in the federal budget.

But lawmakers and aides on both sides of the aisle could not recall Congress ever asking for a criminal investigation of an earmark.

The version of the highway bill approved by the House and Senate in the summer of 2005 mentioned only "widening and improvements" for I-75 in Collier and Lee counties in South Florida. After final passage of the measure, but before it was sent to the White House, that line item was altered to specify that the money would go to "Coconut Rd. Interchange/Lee County."

For months, no lawmaker stepped forward to say who had made the change.

"Somewhere along the way, something changed. Nobody knows for sure who did what," Sen. Tom Coburn (R-Okla.), who fought Young's bridge project three years ago, said during yesterday's debate.

Young's office accepted responsibility yesterday for the change, insisting that campaign contributions were not the motive. Rather, presentations made by Florida Gulf Coast University officials and the developers proved the case for the project, aides said.

Kenny, Young's spokeswoman, said the lawmaker always intended for the earmark to designate money to the interchange project, not generic highway improvements. So committee aides altered the bill to reflect that after the House and Senate had approved it.

"There was an error in the bill and so it was corrected," she said.

Young, who is facing his most difficult reelection campaign since he first won office in 1972, has seen his name surface in connection with other investigations. One of his former aides at the transportation committee has pleaded guilty to accepting gifts from disgraced lobbyist Jack Abramoff. And a former Alaska energy services corporate executive, who pleaded guilty last year to bribery, testified in criminal trials that part of his job was to hold annual fundraisers for Young.

Mack has disavowed any association with the earmark request, and the Florida congressional delegation has worked to place language in "technical corrections" to the highway bill that would allow Lee County to spend the $10 million on general improvements.

The corrections bill is moving through the Senate this week, but Coburn said that is not enough. He is asking for a special House-Senate task force to investigate the origin of the earmark and how it was altered without congressional approval, which could lead to a criminal referral to the Justice Department.

"We ought to be able to investigate ourselves," he said.

However, Boxer and Senate Majority Leader Harry M. Reid (D-Nev.) have suggested that constitutional separation-of-powers issues would make it difficult for a Senate investigation of an action in the House. Instead, they have pushed for a resolution asking the Justice Department to investigate.

A Justice Department spokesman declined to comment.