 -- The FBI and Department of Homeland Security have sent a warning to operators of the nation's power plants -- nuclear and otherwise -- that hackers are targeting such facilities, following a series of hacks that have targeted that critical sector since May.

Administration officials tell ABC News the hacks targeted corporate computers of a number of companies that run nuclear power plants, including the Wolf Creek Nuclear Operating Corporation in Kansas.

Wolf Creek declined to comment on security issues but emphasized that there had been no "operational impact" on its facility. Safety and control systems for the Wolf Creek nuclear plant are not connected to business networks or the internet, according to a company spokesperson.

The New York Times first reported the DHS-FBI warning, which was issued last week.

While nuclear power plant operations were not compromised, the real concern is about the motive of these hacks, which in some cases targeted engineers who worked at the facilities. A critical question is whether the hackers were probing, looking for vulnerabilities or looking for a way in. Authorities say they are unsure.

Sources emphasize that it's still early in the investigation and that the case is ongoing. But cyber forensic experts suspect a nation-state was involved in this hacking. Some officials suspect the hack may have had origins in Russia but again, sources emphasize no final confirmation has been made.