Sen. Barbara Boxer (D-Calif.) said Wednesday afternoon that the Justice Department should take up the investigation over how a controversial earmark for a road project in Florida made its way into the highway technical corrections bill.

Introducing her own amendment to the highway bill, Boxer — the chairwoman of the Environment and Public Works Committee, which spearheaded the bill — said the only way to avoid constitutional and partisan conflicts would be a thorough probe by Justice.

“I think it’s very possible people ought to go to jail here,” Boxer said on the floor, explaining that House and Senate panels can’t send people to jail even if it is warranted.

Boxer’s move comes as there are continuing negotiations over a proposal by Sen. Tom Coburn (R-Okla.) to appoint a special panel to investigate the earmark’s origins. Coburn introduced his amendment on Wednesday, but it’s unclear how much support it can garner after Senate Majority Leader Harry Reid (D-Nev.) filed a cloture motion on the highway bill.

Coburn objected to the idea of sending the probe to the Justice Department, arguing that it is a bad precedent to defer legislative responsibilities to the executive branch.

“We will be setting a precedent that the Congress says that the Justice Department should investigate us,” Coburn said. “I don’t like that precedent. I don’t like it at all.”

Although early in the day Wednesday it appeared that Coburn and Sen. James Inhofe (R-Okla.) had agreed to a deal to have the Government Accountability Office conduct an inquiry into how a $10 million earmark was slipped into the 2005 transportation bill after it had been passed by the House and Senate, that proposal now appears to be in jeopardy because of objections from some Democrats. Inhofe, the ranking member on Environment and Public Works, has acted as a broker between Coburn and Democratic leaders, aides said.

Coburn has led the charge for an investigation into the earmark, which would benefit a Florida land developer connected to Rep. Don Young (R-Alaska), who at the time was chairman of the House Transportation and Infrastructure Committee. Coburn has the support of Florida Sens. Mel Martinez (R) and Bill Nelson (D), as well as that of the three presidential hopefuls: Sens. Barack Obama (D-Ill.), John McCain (R-Ariz.) and Hillary Rodham Clinton (D-N.Y.).

Because cloture will not ripen until Friday morning unless a unanimous consent deal can be reached, Reid’s decision could set the stage for a rare busy Friday for his colleagues, who normally clear out of the Senate late Thursday or early Friday.

The Senate on Wednesday afternoon cast its first vote on the bill since taking it up Monday, handily defeating a motion to recommit the multimillion-dollar measure to Environment and Public Works by a vote of 78-18. The motion had been sponsored by Sen. Jim DeMint (R-S.C.), who has criticized spending in the bill.