Paul Manafort had previously been under FISA surveillance beginning in 2014, although it was discontinued in 2016. Later in 2016, a new FISA warrant re-initiated surveillance on Manafort, which continued into 2017. This second FISA warrant against Manafort was likely obtained due to his intercepted communications with suspected Russian operatives, and other communications between Russians. In addition to surveillance, this warrant also included authorization to search a storage facility belonging to Manafort. Source
In August 2016, Paul Manafort met with Konstantin Kilimnik, a business associate from Ukraine who once served in the Russian army. Kilimnik was suspected of having ties to Russian intelligence and was a person of interest in the Senate Intelligence Committee investigation into Trump–Russia collusion. Source
Also on August 19, 2016, Paul Manafort created the holding company Summerbreeze LLC. Two weeks later, Summerbreeze took out a $3.5 million loan from S C 3, a subsidiary of Spruce Capital (backed by Ukrainian fertilizer magnate Alexander Rovt), using Paul Manafort’s home in Bridgehampton as collateral. The lender did not record the mortgage. The loan was repaid in December 2016. Summerbreeze also received $13 million in loans from two businesses tied to Donald Trump. Source
In February 2017, hackers broke into Manafort’s daughter Andrea’s iPhone, and leaked about 300,000 of her text messages from a four-year period. In a series of text messages between two of Manafort’s daughters, Andrea was extremely harsh in her criticism of Manafort’s dealings in Ukraine:
Don’t fool yourself… That money we have is blood money. You know he has killed people in Ukraine? Knowingly… As a tactic to outrage the world and get focus on Ukraine. Remember when there were all those deaths taking place. A while back. About a year ago. Revolts and what not. Do you know whose strategy that was to cause that, to send those people out and get them slaughtered.
Further texts from Andrea stated that Manafort’s work in, and payments from Ukraine were legally questionable, and that Manafort was cash-poor since Ukraine was late in paying him. Source
In July 2017, federal agents served an early morning no-knock search warrant at Paul Manafort’s residence, picking his lock to gain entry. After seizing evidence and photographing evidence of Manafort’s wealth, they told Manafort they planned to indict him. Source Manafort was, in fact, indicted on October 31, 2017. It was revealed that the FBI raid of his home found evidence that Manafort had withheld from Mueller. Source
On August 15, 2016, 2 days after the New York Times published a report about Manafort’s connections with Yanukovych, and 3 days before Manafort resigned (source), Vladimir Putin met with Viktor Yanukovych in Volgograd, Russia. They discussed recent media revelations about Paul Manafort, and Yanukovych confirmed to Putin that he did authorize and order substantial kick-back payments to Manafort as the reports alleged.
Manafort’s downfall was not only orchestrated by Putin, however. Several senior players close to Trump also wanted Manafort out, to loosen his control on strategy and policy. In particular, Cory Lewandowski, who hated Manafort personally and remained close to Trump.
Following Manafort’s downfall, Trump’s lawyer Michael Cohen’s role and importance grew in the ongoing secret liaison relationship between Trump’s campaign and Russian leadership. One of Cohen’s tasks was to contain further scandals involving Manafort’s role relating to Russia & Ukraine.