Cyberattacks reported at US airports

Some of the nation's largest airports have been targeted for cyberattacks Monday by an attacker within the Russian Federation, a senior official briefed on the situation confirmed to ABC News.

Importantly, the systems targeted do not handle air traffic control, internal airline communications and coordination or transportation security.

"It's an inconvenience," the source said. The attacks have resulted in targeted "denial of public access" to public-facing web domains that report airport wait times and congestion.

Over a dozen airport websites were impacted by the "denial of service" attack, John Hultquist, head of intelligence analysis at cybersecurity firm Mandiant, told ABC News. That type of attack essentially overloads sites by jamming them with artificial users.

"Killnet," a pro-Russian hacker group, is believed to be behind the attack, according to Hultquist. While similar groups have been found to be fronts for state-backed actors, Hultquist said there is no evidence the Russian government was involved in directing this attack.

The attacks were first reported around 3 a.m. ET when the Port Authority notified the Cybersecurity and Infrastructure Security Agency that the LaGuardia Airport system had been hit. LaGuardia has been restored, but other airports around the country have subsequently been targeted.

The FBI and Cybersecurity and Infrastructure Security Agency, part of the Department of Homeland Security, each said they were aware of the attacks.

The websites for Des Moines International Airport, Los Angeles International Airport (LAX) and Chicago O'Hare International Airport appeared impacted Monday morning.

Later Monday, Denver International Airport, the third busiest airport in the country, was attacked and continues to be attacked, according to an airport spokesperson.

"Similar to many other U.S. airports, DEN’s website has been targeted," the spokesperson told ABC News. "The attacks began around 11 a.m. this morning and they continue. The attackers are attempting to overwhelm our website so that it becomes unavailable to the public."

"At this time, the attacks have not been impactful, though we are closely monitoring these attacks and any others. We are also sharing information on these attacks with TSA, CISA and other airports," the spokesperson added.

Hartsfield-Jackson Atlanta International Airport reported around 10:30 a.m. ET that its site is back up and running and that "at no time were operations at the airport impacted."

"Early this morning, the FlyLAX.com website was partially disrupted," LAX said in a statement to ABC News. "The service interruption was limited to portions of the public facing FlyLAX.com website only. No internal airport systems were compromised and there were no operational disruptions."

LAX said its website was back up and running a little before 1 p.m. ET.

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.

Jamming attacks like the one seen Monday morning are highly visible but largely superficial and often temporary, Hultquist said.

"We are pretty clear it's a Russian cyber group that claimed responsibility," Sen. Chuck Schumer, D-N.Y., said Monday, going on to connect the attacks to the Ukrainian bombing of a bridge in Crimea over the weekend. "We are asking our authorities to confirm who did it and then take the appropriate strong action so the Russians know they cannot get away with this. Putin has a lot of nerve, after his brutal vicious war against the Ukrainian people, to now say he has the right to retaliate because they protected themselves with a bridge is outrageous."

ABC News' Alex Stone and Jeffrey Cook contributed to this report.