The same NY Times article told us when, here:
The F.B.I. investigation into Mr. Manafort began last spring, and was an outgrowth of a criminal investigation into his work for a pro-Russian political party in Ukraine and for the country’s former president, Viktor F. Yanukovych. In August, The Times reported that Mr. Manafort’s name had surfaced in a secret ledger that showed he had been paid millions in undisclosed cash payments. The Associated Press has reported that his work for Ukraine included a secret lobbying effort in Washington aimed at influencing American news organizations and government officials.