The board’s report says that the fund-raising effort was run out of Mr. de Blasio’s City Hall and involved Emma Wolfe, his top political aide, and Ross Offinger, a fund-raiser for the mayor’s election campaign as well as the Campaign for One New York, a nonprofit created by Mr. de Blasio to support his agenda. The report also names several union officials and political consulting firms, including BerlinRosen, which has longstanding ties to the mayor.

It says that those people worked closely with the candidates and their campaigns in arranging for large donations to be made through the county and statewide committees, and then designated how the money would be spent. Some of the checks to the statewide committee included notations such as “Donation per Mayor,” the report said.

One of the potentially most damning pieces of evidence cited in the report was in an email from the treasurer for Ms. Tkaczyk’s campaign, Matthew Lerch, to Hayes Clement, the treasurer of the Ulster County Democratic Committee. “Has the check for $60k cleared?” the email from Mr. Lerch said, according to the report. “Below is our banking info, we need the 60 transferred over ASAP please.”

The email was sent on Oct. 16, 2014. State campaign finance records show that the Ulster County committee received a $60,000 contribution from the political action committee of the New York State Nurses Association, a union, on Oct. 14. The records indicate that on Oct. 17, the county committee transferred the same amount to Ms. Tkaczyk’s campaign.

In all, the Ulster County committee transferred $330,000 to Ms. Tkaczyk’s campaign over a few days. Around the same time, the Putnam County committee transferred $367,000 to Mr. Wagner’s campaign and $273,750 to Mr. Gipson’s, in each case after taking in similar amounts from donors outside the county.

“Reasonable cause exists to believe that all the contributions were made to the county committees simply as straw donors,” the report said. “Therefore they should be considered contributions to the candidates and subject to the candidates’ contribution receipt limits.”

Despite the flood of cash, all three candidates lost their races.

Investigators said that similar patterns existed with candidates running in Monroe and Erie Counties.