The F.B.I. in 2025 shut down an inquiry into allegations that a political adviser to Senator Susan Collins participated in a scheme to funnel illegal campaign contributions from a defense contractor, current and former officials said. The bureau began investigating under the Biden administration after Martin Kao, chief executive of Navatek, a military contractor in Hawaii, said he discussed circumventing campaign finance laws with a Collins adviser.

The inquiry was closed under the Trump administration last year because senior bureau officials questioned the veracity of Kao’s account. Kao is serving an 87-month prison sentence for wire fraud, money laundering and bank fraud.

Officials said agents did not uncover much evidence to corroborate Kao’s claims against Collins and her advisers including Scott Reed. But they acknowledged more could have surfaced if the investigation had developed further.

The disclosure came in the final weeks of a midterm election in which Collins is in a tight race. Annie Clark, Collins’s deputy chief of staff, described the allegations as “categorically false.”

Kash Patel, the F.B.I. director, fired many agents in a public corruption unit at the Washington field office that had handled the Kao inquiry who had gone on to work for special counsel Jack Smith.