Then-Trump campaign chairman Paul Manafort at the Republican National Convention. (Matt Rourke/Associated Press)

Randall D. Eliason teaches white-collar criminal law at George Washington University Law School.

Reports that the FBI wiretapped former Trump campaign manager Paul Manafort are a further sign of the seriousness of special counsel Robert S. Mueller III’s investigation. But there’s still a great deal we don’t know about the implications, if any, for the broader inquiry into possible Russian ties to the Trump campaign.

CNN reported Monday night that the FBI obtained a warrant to listen in on Manafort’s phone calls back in 2014. The warrant was part of an investigation into U.S. firms that may have performed undisclosed work for the Ukrainian government. The surveillance reportedly lapsed for a time but was begun again last year when the FBI learned about possible ties between Russian operatives and Trump associates.

This news is a big deal primarily because of what it takes to obtain such a wiretap order. The warrant reportedly was issued under the Foreign Intelligence Surveillance Act. A FISA warrant requires investigators to demonstrate to the FISA court that there is probable cause to believe the target may be acting as an unlawful foreign agent.

With some of the closest members of President Trump's campaign slated to testify before congressional panels investigating its ties with Russia, here's what investigators want to ask Trump's son and former campaign manager. (Jenny Starrs/The Washington Post)

When news broke last month that Mueller was using a grand jury to conduct his investigation, many reported it with unnecessary breathlessness. Although a grand jury investigation is certainly significant, a prosecutor does not need court approval or a finding of probable cause to issue a grand jury subpoena, and Mueller’s use of a grand jury was not unexpected.

A FISA warrant is another matter. It means investigators have demonstrated probable cause to an independent judicial authority. Obtaining a warrant actually says much more about the strength of the underlying allegations than issuing a grand jury subpoena.

That’s also why the search warrant executed at Manafort’s home in July was such a significant step in the investigation. Unlike a grand jury subpoena, the search warrant required Mueller’s team to demonstrate to a judge that a crime probably had been committed.

But it’s important not to get too far in front of the story. The FBI surveillance of Manafort reportedly began in 2014, long before he was working as Trump’s campaign manager. So the initial allegations, at least, appear to have involved potential crimes having nothing to do with the Trump campaign. And most or all of the surveillance apparently took place before Mueller was even appointed and was not at his direction.

Mueller’s involvement now does suggest that the current focus relates to Manafort’s role in the Trump campaign. But we don’t know exactly how, if at all, any alleged crimes by Manafort relate to his work in that role. And we don’t know whether any other individuals involved in the campaign are potentially implicated.

We also don’t know what evidence was obtained as a result of the surveillance. The fact that warrants were issued does not mean any evidence of criminal conduct was actually found.

The other import of this news involves the possible implications if Manafort is charged. The New York Times reported Monday that when Manafort’s home was searched in July, investigators told him he should expect to be indicted. Even if Mueller were to indict Manafort for crimes not directly related to the Trump campaign, it would be a significant development. A typical white-collar investigation often proceeds by building cases against lower-level participants in a scheme — the little fish — and then persuading them to cooperate in the investigation of the bigger fish. Trump and his associates therefore may have reason to be concerned about what Manafort could tell investigators if he were indicted and chose to cooperate.

Again, much of this is speculation. Due to grand jury secrecy and the secrecy surrounding the FISA process, we don’t know many of the details. And given the typical pace of these investigations, whatever happens likely will not happen quickly.

But news of the FISA surveillance is the latest evidence that Mueller’s investigation is serious, aggressive and will be with us for some time.