In the year since the start of special counsel Robert Mueller's investigation, one thing has been notably absent: a public indictment of any Russians for the hacking of the Democratic National Committee (DNC).

Mueller has charged President Trump's former campaign chief, secured guilty pleas from several individuals in Trump's orbit and indicted 13 Russians for an elaborate plot to leverage social media to influence the American electorate.

But the special counsel has yet to announce charges for the hacking of the DNC, even though the intelligence community and private cybersecurity experts linked the attack to the Russian government more than a year ago. Legal experts say there are several possible explanations.

"[The reasons] can range from, there's no evidence of any known individuals, to publicly announcing the indictment would compromise other aspects of the investigation" said Mark Zaid, a Washington-based attorney specializing in national security.

Mueller also might not be able to reveal the "information they might possess to prove the case," Zaid said, because it could compromise intelligence sources or methods.

Experts broadly agreed that the lack of a public indictment should not be interpreted as a sign that charges have not been brought for the hacking.

"There may be one and it may be sealed," said Jack Sharman, a former special prosecutor to Congress for the Whitewater investigation, adding that prosecutors often file indictments under seal when "they don't want to alert targets and potentially useful witnesses that in fact somebody has already been charged."

Reports in recent months have suggested that Mueller's team has compiled enough evidence to charge those suspected for the DNC attack. If no charges have been brought, it's possible the special counsel's team is still building a case against the suspects or wants to understand the full scope of what happened.

"If there are Americans involved, whether or not they have anything to do with the campaign, then I would expect an indictment," said Steven Cash, a lawyer at Day Pitney.

One of the main unanswered questions for Mueller is whether anyone associated with the Trump campaign was involved in the theft or release of the hacked emails.

In recent days, the special counsel has appeared to zero in on former Trump adviser Roger Stone, reportedly subpoenaing two individuals who have worked with the longtime Trump adviser.

Stone has attracted scrutiny for claiming during the campaign to have a line of communication with WikiLeaks, the organization that published troves of hacked emails from the DNC and Clinton campaign chairman John Podesta.

While Stone once claimed in an interview that he had communicated with WikiLeaks founder Julian Assange, he later walked that back, saying it was done through an "intermediary."

Over the weekend, Stone told NBC's "Meet the Press" that he's preparing for the possibility of being indicted.

"It is not inconceivable now that Mr. Mueller and his team may seek to conjure up some extraneous crime pertaining to my business, or maybe not even pertaining to the 2016 election," Stone said. "I would chalk this up to an effort to silence me."

Russia's alleged involvement in the hacking first came to public attention nearly two years ago, when CrowdStrike, a cybersecurity firm hired by the DNC, identified two separate Russian-affiliated breaches of the DNC networks in 2015 and 2016.

Soon after, DCLeaks.com, WikiLeaks and hacker persona Guccifer 2.0 began releasing emails pilfered from DNC servers. In October 2016, the Department of Homeland Security and the director of national intelligence publicly blamed the Russian government for directing the email hacks.

The January 2017 intelligence community assessment offered a more complete picture of the Russian activity. It described an influence operation that married covert cyber operations with disinformation to undermine confidence in U.S. democracy, damage Democratic presidential nominee Hillary Clinton and help Trump win.

It also said that Russian intelligence gained access to DNC networks in July 2015 and maintained it until at least June 2016, noting that the GRU, Russia's main intelligence agency, "had exfiltrated large volumes of data from the DNC" by May 2016.

Still, tying the hacking operation to specific Russian individuals or entities would warrant significant resources and time, likely requiring federal officials to obtain information from overseas to bolster their case.

"Any time you've got to gather evidence from another country, it makes everything that much more complicated," said Randall Eliason, a George Washington University law professor and former assistant U.S. attorney. "Not only physical evidence, like documents, but individuals."

"It may be that there is evidence they never will be able to get, no matter how hard they try," Eliason said.

Recent reports have offered a tantalizing glimpse into Mueller's findings. The Wall Street Journal reported last November that the Justice Department had identified more than six Russian government officials involved in the DNC hack and was weighing whether to indict them.

In March, the Daily Beast reported that federal prosecutors have evidence showing that Guccifer 2.0, the hacking persona who claimed responsibility for the DNC breach, was a member of the GRU.

But even with a wealth of evidence, Mueller would find it nearly impossible to successfully bring a case against Russian intelligence officers who are out of reach of the U.S. government. A public indictment would do little more than send a warning message to Russia - and issuing it would not necessarily be a high priority of the sprawling investigation, which includes looking at whether the president obstructed justice.

Filing an indictment under seal, however, could prove more attractive.

"Sealed indictments are commonplace," Sharman said. "That might also be particularly useful or important when you may only have one shot at a target - a foreign national, somebody who is not subject to jurisdiction readily, someone who may have to travel to a country with whom we have an extradition treaty."

Federal officials opened the investigation into Russian interference in July 2016, and Mueller was appointed special counsel in May 2017, after Trump abruptly fired FBI Director James Comey.

The special counsel's probe offered its first bombshell development last October, in the form of financial-related charges against former Trump campaign aides Paul Manafort and Richard Gates.

The same day, Mueller unveiled a guilty plea from George Papadopoulos, a former Trump campaign adviser who was allegedly told by a Russia-linked professor in April 2016 that the Russians possessed "dirt" on Clinton in the form of "thousands of emails."

Critics of the president have pointed to links between Papadopoulos and other campaign associates and Russia as evidence of the campaign's potential culpability in the interference operation. Last month, the Democratic Party filed a lawsuit against Russia, the Trump campaign and WikiLeaks, alleging a conspiracy to interfere in the 2016 campaign and aid Trump.

"I firmly believe that we will see that indictment, but the only question is will that indictment include U.S. persons or just Russians," said Rep. Adam Schiff (Calif.), the top Democrat on the House Intelligence Committee, who claimed to have seen "overwhelming" evidence underling the intelligence community's assessment that the Russian government was behind the hacking of the DNC.

"I think that Mueller would want to finish the collusion investigation before he charges that because that is really central to the whole issue of collusion - what role did the Trump campaign play in the release of, timing of, content of these Russian stolen materials," Schiff added.

The special counsel's office declined to comment for this article.