Airlines tried social distancing on board. For many, that experiment is ending.

After capping the number of people on flights since April, American Airlines announced Friday that its planes will likely be full in a few days.

"As more people continue to travel, customers may notice that flights are booked to capacity starting July 1," the airline said in a news release. "American will continue to notify customers and allow them to move to more open flights when available, all without incurring any cost."

The carrier said passengers could potentially - if there's room, and if there are no restrictions due to weight or balance - move to other seats in the cabin if they're seated next to someone they don't know.

Reaction to the news was swift and angry at the end of a week that saw record daily numbers of new coronavirus infections in the United States. Two states - Florida and Texas, where American is headquartered - reinstated some restrictions as cases climbed dramatically.

"I find it absolutely appalling you chose to sell the middle row seats as Covid numbers continue to spike," one Twitter user wrote in a message directed to the airline. "I am loyal to AA but clearly you are not concerned with the safety of the public. Looks like I will fly delta since they removed the middle row. EXPECTED BETTER!"

Earlier this week, the union that represents American's pilots pushed a plan that would have the government buy enough seats on each flights so no one would have to sit next to a stranger.

"Passengers would be encouraged to fly more thanks to uniform social distancing, airlines would be encouraged to operate more flights, and the government would ensure preservation of critical transportation infrastructure and related jobs," the Allied Pilots Association proposed.

According to the Transportation Security Administration, 623,624 people went through TSA checkpoints on Thursday - the highest number since March 19.

As the number of travelers has increased, airlines have introduced more safety measures including requiring passengers to wear face coverings.

United said in mid-May that it would "avoid where possible seating customers next to each other" but could make no guarantees. Instead, the airline said at the time that it would "do our best" to contact travelers on flights that were expected to be close to full in case they wanted to rebook on another flight.

United spokesman Charles Hobart said in an email that the policy would stay in place through July 31, but he confirmed that the carrier does not block middle or adjacent seats.

Delta is still blocking middle seats and has committed to capping seating at 50 or 60%, depending on the part of the plane - at least through Sept. 30. Southwest Airlines is blocking about a third of the seats on its planes from being booked, which it says allows for middle seats to stay open. That cap is also in place until at least Sept. 30.

JetBlue said it would continue blocking all middle seats on larger planes and aisle seats on smaller aircraft through at least July 31.

“You’re going to definitely have to sit next to a stranger again, I’m afraid, on a plane,” JetBlue chief executive Robin Hayes said during a Washington Post Live discussion in late May. “Because [of] the economics of our industry, most airlines have a break-even load factor of 75 to 80%, so clearly capping flights at 55 to 60%, which is what we’re doing right now . . . is not sustainable.”