Washington (CNN) President Donald Trump on Monday accused the Obama administration of using the investigation into potential ties between his campaign and Russia's meddling in the 2016 election to discredit his bid and boost the chances of his Democratic opponent, Hillary Clinton.

"Why did the Obama Administration start an investigation into the Trump Campaign (with zero proof of wrongdoing) long before the Election in November? Wanted to discredit so Crooked H would win. Unprecedented. Bigger than Watergate! Plus, Obama did NOTHING about Russian meddling," Trump tweeted Monday morning.

Why did the Obama Administration start an investigation into the Trump Campaign (with zero proof of wrongdoing) long before the Election in November? Wanted to discredit so Crooked H would win. Unprecedented. Bigger than Watergate! Plus, Obama did NOTHING about Russian meddling. — Donald J. Trump (@realDonaldTrump) March 5, 2018

It's unclear exactly what pre-election investigation Trump was referring to, though the FBI opened its investigation into the Trump campaign's links to Russia in July 2016. Then-FBI Director James Comey oversaw that investigation until Trump fired him in May 2017. Shortly after that, former FBI Director Robert Mueller was appointed as special counsel to continue the probe.

Former Trump campaign chairman Paul Manafort was wiretapped before and after the election. Some of the intelligence collected includes communications that sparked concerns among investigators that Manafort had encouraged the Russians to help with the campaign, three sources familiar with the investigation have told CNN. Two of these sources, however, cautioned that the evidence is not conclusive. Manafort was Trump's campaign chairman from May to August 2016.

The FBI also eavesdropped on Carter Page , a campaign associate that then candidate Trump once identified as a national security adviser, on suspicions he was acting as a Russian agent. Page stepped away from the campaign in September 2016 amid questions about his Russian ties.

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