Former Tennessee Gov. Phil Bredesen's campaign for U.S. Senate told the FBI on Thursday that it fears it has been hacked, amid growing concern that candidates in the 2018 election could be targets of cyberattacks.

In a letter obtained by The Associated Press, campaign lawyer Robert E. Cooper Jr. wrote that Bredesen's aides became suspicious when someone pretending to be the campaign's media buyer asked for money to be wired to an international account.

The letter says the person used an email address nearly identical to the actual media buyer's and knew about an upcoming TV campaign and its proposed dates. Cooper says the campaign hired a cyber-security firm that found the impostor emails were registered through an Arizona-based registrar.

"Thanks to alert action by campaign management, no funds were diverted," the letter states. "However, due to the fact that the impostors knew the media buy was imminent, we are concerned that there has been an unauthorized intrusion into the extended campaign organization."

The possible hack comes amid warnings from U.S. officials that the 2018 midterm elections could face additional cyber meddling from Russia and others, from both domestic and international sources.

It's unclear who may be behind the suspected Bredesen campaign breach or what the motive may have been.

The letter says the campaign's internal review is ongoing. The campaign has since bought the TV ad time and the spot is running.

Additionally, Bredesen sent an email to his contact list Thursday telling people not to open a document in a Wednesday email that appears to be from him. He wrote that the fake emails have the domain "bredesens.com," compared to his "bredesen.com" domain.

"On Wednesday afternoon (March 7) you may have received an email, coming from a spoofed email address, that appeared to come from me, asking you to click a link to view a shared document," Bredesen wrote. "Unfortunately, this email is part of a cyber intrusion into my extended campaign organization, including an unsuccessful attempt to divert campaign funds to offshore accounts. The FBI has been contacted and is involved."

"I'm sorry for this inconvenience; this sort of thing is rapidly becoming one of the unpleasant facts of life," the Bredesen email concludes.

Bredesen campaign spokeswoman Laura Zapata said, "We have no further comment at this time."

Bredesen is the likely Democratic nominee for the open seat in Tennessee to be vacated by Republican Sen. Bob Corker. U.S. Rep. Marsha Blackburn is running for the Republican nomination.

The race could prove critical in the balance of the U.S. Senate, where Republicans currently hold a 51-49 advantage.