Washington CNN  — 

United Airlines says it is canceling flights through “early July” because of the Boeing 737 Max grounding.

The airline joins other US carriers — American and Southwest — that have canceled 737 Max flights to avoid disruption as people book flights for the upcoming travel season.

“We’ve used spare aircraft and other creative solutions to help our customers … get where they are going,” United spokesman Frank Benenati said in a statement Monday. “But, it’s harder to make those changes at the peak of the busy summer travel season.”

All of Boeing’s Max planes were grounded worldwide last month after a 737 Max jet flown by Ethiopian Airlines pilots crashed in that country, killing everyone aboard. It was the second fatal crash involving a Max in recent months.

United (UAL) said the initial decision to take the planes out of service affected “roughly 40 flights a day” at the time. The company doesn’t fly any Max 8 jets, the type of plane that was involved in both of the fatal incidents. But it does have 14 Max 9s, which are a slightly longer version of the Max 8.

Other US airlines have even more Max planes in their fleets. Southwest (LUV), which flies 34 Max jets, said last week that it would cancel Max flights through August 5. American (AAL), which has 24 of the planes, said Sunday it would cancel about 115 daily flights through August 19.

The Federal Aviation Administration held a meeting Friday with officials from the three US airlines. The FAA said authorities discussed input from 737 Max pilots and operators “as the agency evaluates what needs to be done before the FAA makes a decision to return the aircraft to service.”

Investigators are still probing the cause of the Ethiopian crash, as well as the other Max crash involving a Lion Air jet last October.

The focus of the crash investigations is the plane’s automatic safety system, for which Boeing says it is developing a software fix. Last Thursday, Boeing CEO Dennis Muilenburg said about two-thirds of the more than 50 customers from various airlines have been able to test the software patch using a flight simulator.