Air traffic controllers’ radio at Denver International Airport experienced an outage for a minute-and-a-half this week, according to the Federal Aviation Administration (FAA).

The federal agency said a part of the Denver Air Route Traffic Control Center suffered a loss of communications for about 90 seconds at around 1:50 p.m. local time Monday because “both transmitters that cover a segment of airspace went down.”

Controllers utilized another frequency to relay instructions to pilots flying in, according to the FAA. Aircraft remained “safely separated” and there were no “impacts to operations.”

The FAA is investigating the incident.

As many as 20 pilots flying aircraft into the airport were not able to communicate with air traffic controllers Monday, Denver7 reported Thursday, citing multiple unnamed sources.

The outage marks another incident at U.S. airports in recent weeks. Newark Liberty International Airport has been in the headlines lately over experiencing multiple delays while dealing with staffing challenges, mostly among air traffic controllers, and outdated technology.

One of the New York City metropolitan area’s biggest airports has also been set back with construction on parts of the runway, which has contributed to delays.

Transportation Secretary Sean Duffy said last weekend that the number of flights out of Newark Airport has been reduced, but he argued that it is still safe to fly out of the airport.

Duffy told lawmakers during a House Transportation subcommittee hearing that the FAA “has brought in together all of the airlines who serve Newark to have a conversation about how there can be a delayed reduction.”

“So, if you book your flight, that flight is going to fly. You don’t have people at the airport for, you know, two, four, six hours, then a flight canceled,” Duffy said Wednesday. “So we’re working through that now, hopefully in the next week or two, we’ll have additional really good news about the telecom progress that we’ve made.”