US-Russia probe filing suggests broader case against Trump ally Roger Stone

The investigation into President Donald Trump's longtime political consultant Roger Stone could broaden from the charges of lying, obstruction and witness-tampering on which he was arrested last week, according to a court filing by Special Counsel Robert Mueller Thursday.

The prosecutor asked the federal court in Washington to give his team and the defence more time to review "terabytes" of evidence, including multiple communications devices and hard drives, email and cloud computing accounts, and bank and financial records going back years.

Investigators already appeared to have sufficient email and witness evidence to press last Friday's indictment of Stone on five counts of lying to Congress over his contacts with WikiLeaks, and one count each of obstruction and witness tampering.

The need to sift through that much more material suggested that Mueller, who leads the Russia meddling probe, is casting his net wider in terms of offences and investigation targets.

"Perhaps they want Stone to have this information now because there could be additional charges down the line, or because they think his knowledge that they possess this information could encourage him to flip," said former federal prosecutor Renato Mariotti.

Mueller has charged six former aides of Trump, including his lawyer Michael Cohen, campaign chair Paul Manafort, and national security advisor Michael Flynn, in the 20-month-old investigation into whether the 2016 Trump campaign colluded with Russians.

All have been accused of lying to investigators, and some also of financial and tax-related charges.

But none has yet been charged with any crime involving collusion with Russia, and Mueller has given no indication on whether or not any such charges could be forthcoming.

The Stone case is the closest Mueller has gotten to suggesting there was an illegal conspiracy to collude — yet that was not among the charges.

Stone allegedly lied about contacts with WikiLeaks, the anti-secrecy website that published embarrassing documents from Trump's election rival Hillary Clinton that were allegedly hacked by Russian intelligence.

In a news conference Thursday, Stone said his alleged lies were simply memory lapses and were immaterial to the Russia collusion investigation.

"I'm not accused of collaboration, I'm not accused of conspiracy," he said.

"There is no underlying crime."