Special prosecutor Robert Mueller has secured at least one indictment in his wide-ranging investigation into possible collusion between the 2016 election campaign of President Donald Trump and the Russian government.

A federal grand jury handed down the indictment on Friday, but a federal judge ordered it sealed; so it remains unclear who has been indicted or what charges are involved. Media reports suggest the indictment could be unsealed as early as Monday, with one or more suspects taken into custody.

US intelligence agencies concluded in January that the Kremlin interfered in the 2016 presidential election with the aim of helping Republican candidate Donald Trump defeat the Democratic challenger, Hillary Clinton.

The interference appeared in the form of an electronic hacking operation that broke into the email account of at least one senior Clinton campaign official. Emails that were deemed embarrassing were released to the public via social media as part of a detailed propaganda campaign aimed at discrediting her candidacy.

Mueller, the special prosecutor and former director of the FBI, is trying to determine if Trump's campaign team colluded with the Russians in their efforts.

Mueller was given sweeping powers to investigate possible wrongdoing when he was appointed by Deputy Attorney General Rod Rosenstein in May.

"If the Special Counsel finds it necessary and appropriate, the Special Counsel is authorized to prosecute federal crimes arising from the investigation of these matters," Rosenstein wrote in a letter appointing Mueller back in May.

Peter Carr, a spokesman for Mueller has declined to comment on the indictment.

Trump cries "witch hunt"

Trump has denied allegations that there was any collusion between his campaign and Moscow. He has slammed the special prosecutor's investigation, calling it "a witch hunt."

The Kremlin has likewise denied allegations of wrongdoing.

Mueller has used his broad authority to probe links between the Trump campaign and foreign governments as well as possible money laundering, tax evasion and other financial crimes.

Mueller is also trying to determine whether Trump or any of his staff tried to obstruct justice. Mueller was appointed to take over the criminal probe a week after Trump fired FBI director James Comey, who was then investigating Trump and his possible ties to Russia.

Trump initially said he fired Comey because his leadership of the FBI was damaging morale. Subsequently, however, Trump cited "this Russia thing" as his reason for sending the FBI head packing.

Depending on the circumstances, firing the head of the FBI to head off an investigation is potentially "obstruction of justice," which would be a crime.

FBI agents raided the home of former Trump campaign manager Paul Manafort in July. Manafort once did work for, and has financial and real estate dealings with, a pro-Russian political party in Ukraine. Those ties are under investigation, and Manafort is considered a leading target of Mueller's investigation.

Former General Michael Flynn, who served as Trump's national security adviser for three weeks at the start of Trump's presidency, is also a target of Mueller's probe due to business ties to Russia and Turkey.

bik/jm (Reuters, AFP, dpa)