WASHINGTON—A requirement that people on planes and other forms of transportation in the United States wear masks to prevent the spread of COVID-19 will remain in place through at least Sept. 13, the Transportation Security Administration said Friday. The 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.” The rule requires people to wear a mask to help prevent the spread of COVID-19 across all transportation networks in the United States. That includes at airports, on buses and on trains. People under 2 and with certain disabilities are exempt. Penalties for violating the rule start at $250 and can go up to $1,500 for repeat offenders. The Centers for Disease Control and Prevention has said that fully vaccinated people can travel but its guidelines still …