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Questions about President Donald Trump's ties to Russia began during his campaign and have persisted into his young administration, tripping up members of his inner circle. The latest to come under scrutiny is Attorney General Jeff Sessions, who Thursday afternoon withdrew from any federal probes involving the Trump campaign.

While there are many tentacles to the controversy ─ Trump's glowing talk about Russian President Vladimir Putin, questions about his business interests in Russia, aides' ties to the former Cold War adversary ─ the central issue is Moscow's alleged role in attempts to undermine the presidential election and discredit Trump's rival, Hillary Clinton.

That case, under investigation by the FBI, has intensified amid claims that members of the Trump campaign were in contact with Russian officials before and after the election. U.S. investigators have been using intelligence databases to hunt for evidence of the Russian election hacking operation and any collusion by Americans, including Trump associates.

Related: Trump’s Russia Crisis: A Timeline

The first member of Trump's inner circle to fall victim was Michael Flynn, who resigned as national security adviser last month after admitting to post-election phone conversations with Russia's ambassador to the United States, Sergey Kislyak, and misleading Vice President Mike Pence about them. The calls happened on the same day in December that then-President Obama announced sanctions against Russia for the election-related hacking ─ a topic the two men reportedly discussed.

Now a similar predicament has befallen Sessions, who was one of the earliest elected officials to endorse Trump and served as a campaign adviser. Session's decision to recuse himself followed Republicans calling for him to take himself out of the Russia probe, while some Democrats said he should go further and resign.

Here's an overview of the Sessions controversy, and its potential implications.

Why is Sessions under fire?

The Alabama Republican's problems stem from his January confirmation hearings, when he testified under oath to his colleagues on the Senate Judiciary Committee.

On Jan. 10, Sen. Al Franken, D-Minnesota, asked Sessions what he would do if he learned that "anyone affiliated with the Trump campaign communicated with the Russian government in the course of this campaign."

Sessions replied: "I’m not aware of any of those activities. I have been called a surrogate at a time or two in that campaign, and I did not have communications with the Russians, and I'm unable to comment on it."

A week later, Sessions responded in writing to followup questions from Sen. Patrick Leahy, D-Vermont.

Leahy asked: "Several of the President-Elect’s nominees or senior advisers have Russian ties. Have you been in contact with anyone connected to any part of the Russian government about the 2016 election, either before or after election day?"

Sessions responded: "No."

Why do those statements matter?

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Sessions' answers have drawn scrutiny amid revelations this week that he met with Kislyak twice during the presidential campaign ─ once at a public event in July that also included other ambassadors, and again in September during a private meeting in Sessions' office. The meeting was first reported by The Washington Post.

The meeting took place just after news broke that the U.S. was investigating Russian attempts to undermine the election, and as the Obama administration was publicly warning its rival to back off. At the time, Sessions was a member of the Senate Armed Services Committee.

Sessions has said that he has never met with any Russian operatives or discussed the campaign with them, and that any allegations that he had done so were "false."

He said during a press conference on Thursday that he didn't remember much of his September conversation with Kislyak. He recalled talking about terrorism and Russia's actions in the Ukraine, in which things "got to be a little testy." But Sessions said he didn't remember talking about the campaign.

He acknowledged that he should have mentioned the meeting to the Senate committee.

Related: Sessions Met With Russian Ambassador but Didn’t Mislead Senate: Spokeswoman

The recusal helps steer clear of questions about Sessions' ability to oversee the Russia probe, in which he could now be considered a potential witness.

Some have accused him of deliberately misleading the committee. Others have said he should be charged with perjury.

At the same time, some of Sessions' defenders, including White House Press Secretary Sean Spicer, have said Sessions was honest with the committee and is now the victim of a political hit job by the Democrats.

Did Sessions break the law?

It's not illegal for senators to meet with foreign ambassadors.

Sen. Claire McCaskill, D-Missouri, and Sen. Joe Manchin, D-West Virginia, both acknowledged Wednesday that they had met Kislyak.

And it does not appear at this point that Sessions could be found to have committed perjury, experts said.

The law requires showing that someone intended to give a false statement, which in this case would require an analysis of what Sessions believed the questions meant, said Stan Brand, a former general counsel to the House of Representatives who has represented high-profile government officials in public corruption cases.

"The burden for proving that is high," Brand said.

Richard Painter, a former White House ethics officer under President George W. Bush, said Sessions could argue credibly that he thought questions were limited to whether he discussed the campaign with Russian officials.

Nonetheless, Painter said, "It was a very misleading answer." He called it "perjury light," and said it raised questioned about Sessions' ability to effectively serve as attorney general.

What questions remain?

It is still unclear what Sessions discussed with Kislyak, although either side could have recorded it or taken notes.

"As long as the conversation remains unknown, people will still be suspicious of what was said, whether that's merited or not," said Robert Walker, a former chief counsel to Senate and House ethics committees.

But this is bigger than just Sessions, or Flynn, Painter said. Investigators need to find out about anyone involved with Trump who spoke to Russian officials before he was inaugurated. Short of that, he said, Russia potentially could use those conversations to its advantage.

So far, investigators have found information showing contacts between Trump associates and Russians, including Russians linked to the Kremlin, NBC News has reported. Some of the information came from "routine intercepts" that normally might never have been examined, the source close to the investigation says.

It's unclear whether that is how the information about the Sessions meetings came to light, but it has become clear that the Russian ambassador was under FBI scrutiny and his communications were being monitored.

No. But the FBI investigation remains open.