WASHINGTON -- Charges against 21 activists who staged a sit-in at Department of the Interior headquarters last month were dropped by the U.S. Attorney's office earlier this week, a move that will embolden more confrontational protests in the future, proponents of climate protection say.

The DOI protesters received notice at the Tuesday hearing that the U.S. Attorney's office for the District had "declined to proceed with prosecution against you for the the incident that led to your arrest for the offense of unlawful entry."

Ann Wilcox, an attorney with the progressive National Lawyers Guild and one of two attorneys who had agreed to represent the defendants, said the government could have dropped the charges for a variety of reasons: administrative hurdles, a wariness about pulling cops off their beats to testify, the nonviolent nature of their crimes or the diversity of the activists, among others. But ultimately, she said, "we don't know for sure why some cases are dropped and some are processed."

The U.S. Attorney's office in D.C. declined to comment on the specifics of the case.

The group of protesters was arrested at the close of Power Shift, a biennial gathering of climate activists in the nation's capital. On April 18, conference organizers led a demonstration of some 3,000 protesters to the Chamber of Commerce building and the lobby shop of BP. After that rally concluded, hundreds of the marchers joined an impromptu march on the DOI headquarters led by confrontational climate activist organizations Peaceful Uprising and Rising Tide. At the Interior building, some 300 protesters -- including a brass band -- stormed inside the main foyer of the headquarters and staged a raucous protest that went on until the last hold-outs in the entryway were finally arrested some two hours later.

The protest was intended to highlight the role two Interior agencies play in setting the rules for mining and oil companies, Rising Tide spokeswoman Henia Belalia told The Huffington Post at the time. Lax regulation at the corrupt Minerals Management Service -- renamed the Bureau of Ocean Energy Management, Regulation and Enforcement in the wake of the Deepwater Horizon explosion -- helped set the stage for last year’s BP oil spill. The other agency, the Office of Surface Mining, angered activists shortly before the Power Shift conference by opening up over 7,000 acres of federal land in Wyoming’s Powder River Basin to coal extraction.

Climate activists are largely relieved by the local prosecutor's decision to drop the cases against the DOI protesters. A smaller group, however, views the move as an invitation for further confrontational protests.

"Essentially, the United State government told the activists who risked their freedom on April 18th that they’re not willing to argue with the necessity of their actions," Peaceful Uprising member Deb Henry wrote on her blog Tuesday. "The government gave in. They do not want to spend time or money taking activists to court to dissuade other activists from taking action."

But it's not quite as simple as that, as Henry later explained to The Huffington Post: She is still facing charges for a similar protest staged in the House gallery.

Henry and eight other protesters were arrested on April 15, the day before Power Shift began, for singing a modified version of "The Star Spangled Banner" during a series of contentious budget votes in the House.

During a status hearing scheduled for May 31, the plaintiffs will likely have to decide whether to accept the government's diversion offer -- 40 hours or so of community service and six months arrest free to make the charges disappear, Wilcox speculated -- or proceed to a jury trial. If convicted in court, the activists could face up to six months in jail, Wilcox said.

"What they're trying to do is deter these very young people who have no criminal record whatsoever from taking that action in the future," Peaceful Uprising Director Flora Bernard said, referring to the different approaches the government has taken with the two groups of protester arrested during Power Shift. "I really doubt that that's going to work out," she added.

Henry is one of a handful of activists arrested during Power Shift who are not letting their brushes with the law stop them from attending what many in the climate movement view as the next big battle: the march on Blair Mountain this coming June.

Brandon Nida, one of the march's organizers and native West Virginian, said he expected thousands of people will converge on the tiny town of Blair, W.Va., for the protest, which plans to highlight the destruction caused by of mountaintop removal, an environmentally destructive surface mining practice where vegetation and earth are blasted away to expose the coal underneath. Since 1991, six mountaintop removal mining permits have been issued around Blair, Nida said, although no mining has yet occurred.

But Blair Mountain's significance to the protest extends past the recent attempts to mine the mountain. Ninety years ago, Blair Mountain was the site of the Battle of Blair Mountain, one of the largest labor uprising in American history. Although the over 10,000 coal miners seeking unionization lost in a violent battle against coal mine operators -- which were even supported by federal air power -- many consider the battle an important catalyst for the early 20th century labor movement.

Nida and other's hope their June protest will prove a similar catalyst for the current American labor and climate movements.

"With the Wisconsin thing and the Ohio union-busting," Nida said, "people are more receptive when you talk about this huge battle between labor and coal operators or corporations. … What happened at Blair Mountain and central Appalachia was integral to the American labor movement."

"As far as in the environmental movement, mountaintop removal has been really shaking things up," Nida added. "A lot of people are concentrated on what is one of the most most visceral and visible environment devastating practices."

Confrontational protests may play a role in culmination of the five-day demonstration. After the DOI protesters charges were dropped, Henry wrote that there is a "window of opportunity for our movement to engage in more acts of nonviolent civil disobedience." The March on Blair mountain is the next opening, activists say.

"I'm not sure how much organization is going for arrestable actions" at Blair Mountain, said Bernard, who is planning to attend the march. But she predicted that the combined actions of the thirty protesters arrested at Power Shift "are probably going to invigorate and inspire a lot of potential activists to be willing to take risks at Blair Mountain. … I like to think that people are feeling like they're in a comfortable place to risk arrest."

Wilcox agreed that some activists might try to get arrested at Blair Mountain. But the lawyer noted, "they do have the right to do that."