In a nearly party-line vote, Republicans in Congress easily passed a measure on Thursday citing Attorney General Eric Holder with contempt for refusing to produce documents tied to Operation Fast and Furious. But what comes next?

"In the days and weeks to come, we will use what we can in the way of other tools to glean additional information," said Darrell Issa, chairman of the House Oversight and Government Reform Committee, who led the 18-month probe of Fast and Furious. "We will find the truth."

Exactly what Issa hopes to still learn about Fast and Furious, which the Department of Justice already said allowed high-powered weapons to slip into the hands of criminals, remains unclear. After 18 months, no evidence has emerged showing Holder, his top lieutenants, or any White House officials or staff authorized the discredited tactics of Fast and Furious, or deliberately concealed information about those tactics from Congress.

Issa's failure to secure any damning evidence against top officials suggests that his investigation has largely run its course, said Charles Tiefer, a professor at the University of Baltimore Law School.

"This contempt vote is not the beginning of something. It's the end of something," Tiefer said. "It's the last little bit of life that can be squeezed out of the Fast and Furious investigation."

Tiefer, who served as chief counsel to three Democratic House speakers in the 1980s and 1990s, testified as an expert during a Fast and Furious hearing at Issa's invitation.

Documents and testimony overwhelmingly indicate that the "gun-walking" tactics of Fast and Furious were conceived and executed by field agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives and supervised by the federal prosecutor's office in Phoenix, with limited involvement by senior Justice Department staff.

Emails exchanged after the Republican probe in Fast and Furious began in early 2011 further show that DOJ officials were repeatedly told by ATF officials that no gun-walking occurred during the operation. Those denials were transmitted to a Republican senator then retracted.

In a hearing Wednesday, Issa said that documents being withheld by Holder cover internal deliberations over congressional and media inquiries, and date from February 2011 to today -- after Fast and Furious already had come to an end. Issa said he had no evidence indicating a cover-up by Holder or White House officials, or indicating their involvement in authorizing or managing the botched operations.

The former ATF director, Kenneth Melson, was forced to step down last year and other agents and officials directly involved in Fast and Furious have been reassigned pending the outcome of an internal Justice Department investigation.

Shortly before Thursday's contempt vote, the Associated Press reported that documents recently offered by Holder to Issa's committee dated February 2011 show the attorney general aggressively seeking answers on what went wrong in Fast and Furious.

"We need answers on this. Not defensive BS. Real answers," Holder wrote to a subordinate after seeing a story by CBS alleging gun-walking by ATF.

The documents offered to Issa also show senior officials with the Bureau of Alcohol, Tobacco, Firearms and Explosives sought to convince Holder and other Justice Department officials that "gun-walking" never occurred in Fast and Furious. Those assertions appear to have been questioned by Holder.

White House officials claim that these and other internal emails are protected by executive privilege, but said they were willing to disclose many of them in a show of good faith to the oversight panel. The offer was rejected.

Some open-government advocates have joined Republicans in denouncing the White House's invocation of executive privilege, calling it unnecessary and a slap in the face to transparency. "The president cannot assert this power merely to avoid the release of potentially embarrassing or politically inconvenient details," wrote Mark J. Rozell and Mitchel A. Sollenberger, both political science professors and experts on executive power, in an essay for CNN.

But before the contempt vote, Elijah Cummings, the ranking Democrat on the oversight committee, said Issa's refusal to negotiate with Holder over the documents reeked of politics.

"I believe this attorney general has bent over backwards trying to accommodate us, trying to provide the information," Cummings said.

Other Democrats accused Issa of relentlessly targeting Holder during his investigation despite reams of evidence pointing away from his direct involvement, and stoking the fears of gun-rights advocates with allegations that Fast and Furious was directed by the White House as a secret plot to institute gun control.

"This is Darrell Issa's great white whale," Rep. Mike Quigley, (D-Il.), said in an interview before the contempt vote.