After cyberattacks targeted over a dozen U.S. airports Monday morning. By Monday afternoon, their websites were back online.

Airports in New York, Chicago and Los Angeles were impacted by a denial-of-service attack, which overloads sites by jamming them with artificial users. It did not affect transportation security or air traffic control.

An intelligence official telling ABC News that “Killnet”, a pro-Russian hacker group, is believed to be behind the attack. But there is no evidence that the Russian government is involved.

The group “Killnet” has been active since the beginning of the war in Ukraine, targeting Ukrainian allies and recently claiming credit for taking down government websites in the U.S. They operate internationally and have been known to carry out attacks across Europe, according to cybersecurity experts.

Engineers and programmers are actively working to close backdoors that allowed the attacks and shoring up more critical computer infrastructure.