A requirement that people on planes and other forms of transportation in the U.S. wear masks to prevent the spread of COVID-19 will remain in place through at least Sept. 13, the Transportation Security Administration said Friday.Related video above: More Americans booking flights, still down from before pandemicThe mandate, which went into effect Feb. 1, was set to expire May 11. Darby Lajoye, the acting TSA administrator, said in announcing the extension that masks "remain an important tool in defeating this pandemic" even though about half of U.S. adults have received a vaccine. The rule requires people to wear a mask to help prevent the spread of COVID-19 across all transportation networks in the U.S. That includes at airports, on buses and on trains. People under 2 and with certain disabilities are exempt. The Centers for Disease Control and Prevention has said that fully vaccinated people can travel but its guidelines still require the use of masks, social distancing and other measures to prevent the virus.The TSA says it has received reports of 2,000 people who violated the rule, which took effect February 2. The agency also recently began sending citations to alleged violators, an official told CNN earlier this month. The official could not say how many have been issued. There are multiple investigatory and review steps before a citation is ultimately issued.Penalties for violating the rule start at $250 and can go up to $1,500 for repeat offenders.Flight attendant groups that had pushed for the mandate applauded Friday's move.The Association of Flight Attendants said it "will keep passengers and aviation workers safe," and the Association of Professional Flight Attendants called wearing a mask "the simplest way we can safeguard passenger and crew safety."CNN contributed to this report.