A trail of documents has reportedly led Attorney General William Barr's handpicked federal prosecutor to focus his inquiry into the origins of the Russia investigation on the first several months of President Trump's tenure.

John Durham, a U.S. attorney from Connecticut, is zeroing in on the period spanning from January 2017, when Trump took office, to May of that year. A "strong" paper trail, as CBS News senior investigative correspondent Catherine Herridge put it on Friday, has led the investigation into possible misconduct by federal law enforcement and intelligence officials to that time frame.

Durham's office declined to comment for this report.

While Trump and his allies have championed Durham's effort, Democrats have dismissed the allegations of wrongdoing during the Trump-Russia investigation and are concerned the inquiry may be an effort to discredit the work of special counsel Robert Mueller. Trump gave Barr full declassification authority for the endeavor.

Barr and Durham have traveled around the world for the investigation, and Durham's team has already asked witnesses about possible anti-Trump bias among former FBI officials. The secretive DOJ inquiry includes scrutiny of former CIA Director John Brennan, former Director of National Intelligence James Clapper, former FBI special agent Peter Strzok, and British ex-spy Christopher Steele.

In October, it was reported that Durham was expanding the scope of his investigation, adding agents and resources, to examine the post-election timeline up to the appointment of Mueller as special counsel in May 2017. The "investigation into the investigators" was reported to be upgraded to a criminal inquiry later that month, which would give Durham the power to impanel a grand jury and hand down indictments. Durham has also reviewed the Intelligence Community's conclusions about Russian interference in the 2016 election.

Little else is known about the investigation other than that Durham is exploring whether a crime was committed by Kevin Clinesmith, a former FBI lawyer who was found by the Justice Department Inspector General Michael Horowitz to have altered a document during the FBI's efforts to obtain a Foreign Intelligence Surveillance Act warrant renewal to continue wiretapping onetime Trump campaign adviser Carter Page.

Among those known to be cooperating with Barr is retired Adm. Michael Rogers, the former director of the National Security Agency who has a history of uncovering FISA violations.

The period of time under scrutiny by Durham also covers a leak to reporters that federal prosecutors in D.C. are investigating. The Russian intelligence document under scrutiny, word of which made its way into press reports in the spring of 2017, factored into former FBI Director James Comey's handling of the FBI investigation into former Secretary of State Hillary Clinton's email server, and Comey himself appears to be the focus of that inquiry. Comey was fired in May 2017, after which Mueller was appointed special counsel to lead the Russia investigation.

During an interview with NBC News in December, Barr said Durham's investigators are "looking at the whole waterfront," and questions remain unanswered following the release of Horowitz's FISA report. The attorney general pointed to the "problem" of Comey refusing to allow his security clearance to be temporarily reinstated, which allowed him to avoid the questions about classified information that Horowitz wanted to raise. Durham has the ability to compel testimony from other agencies and countries, unlike Horowitz in his limited role as inspector general.

"We have to be careful about the way we collect evidence. And we have to make sure that we have enough evidence to justify our actions. And we're not going to cut corners in that respect,” Barr said before providing a hint about when Durham's investigation may end. “You know, there's some people who think this thing is going to drop in a few weeks. That's not the case. I see this, perhaps, reaching an important watershed perhaps in the late spring, early summer.”