What would happen if the United States were about to be hit by nuclear weapons? Hopefully, the government would provide some warning through the federal alert system. But that system hasn’t always been in place—and it hasn’t always worked. For over 40 minutes on February 20, 1971, a test gone awry led to widespread panic that America had been plunged into nuclear war.

Since 1951, the United States has had a way to use existing TV and radio stations—and now cell phones—to broadcast information about emergencies. The system has its roots in Cold War-era nuclear fears. As relations between the United States and the USSR chilled, defense officials decided to create a system that would not just allow officials to transmit information to the American people, but to confuse potential Soviet aircraft. CONELRAD was designed to quickly switch off American radio stations, then activate select ones to transmit civil defense information in an attempt to prevent aircraft flying over the U.S. from using radio signals to navigate.

In 1963, the system was upgraded to the Emergency Broadcast System. This new system could be used for national emergencies and to transmit information about weather and natural disasters on a local level. If a national-scale emergency were to occur, an emergency alert would go to the entire nation from the National Warning Center inside NORAD, the nation’s aerospace defense command center located deep inside Cheyenne Mountain in Colorado Springs. Once the EBS was activated, the president could speak to the entire nation within 10 minutes.

At least, that was the plan.

The system was tested every Saturday. But on that Saturday in 1971, a real message—not a test message—came over the special teletype network that sat inside every radio and TV station. “Message authenticator: hatefulness, hatefulness, it read. “This is an emergency action notification (EAN) directed by the President. Normal broadcasting will cease immediately.”

“Hatefulness” matched up with the special daily code words sent to broadcasters to verify an emergency alert. This wasn’t a drill.

Immediately, broadcasters sprang into action. They cut into their normal programming and read a special, federally mandated script that told the audience they were interrupting their regular programs at the government’s request. But though the voices that told local audiences to tune into a station that was broadcasting news and information were calm, the stations broadcasting them were anything but.

“The composure of portions of the broadcast industry—and the country—was in shambles,” wrote Variety. “Some stations broadcast the announcement and went off the air as required—throwing listeners into a tizzy. Other stations didn’t pick up the warning until after it had been cancelled. Some went off the air without having the nerve to broadcast the warning.”

Chaos reigned outside radio stations, too. Frantic listeners called their local stations to find out what was going on. Others huddled around TV sets, fearing the worst. At the time, the Vietnam War was raging. Had the United States’ ongoing battle against Communism finally resulted in nuclear war?

Meanwhile, confused authorities struggled to make sense of what was happening. Officials called the Pentagon demanding an explanation. When the warning center realized what it had done, employees scrambled for the code word they needed to stop the broadcasts. They couldn’t find it. They tried to cancel the broadcast six times. Each time, they failed.

Finally, more than 40 minutes after the first transmission, the Office of Civil Defense sent a cancellation message with the right code word—“impish”—to broadcasters. The first major test, and failure, of the Emergency Broadcast System was over. Programming resumed and the American people breathed a sigh of relief.

The Office of Civil Defense explained that an operator at the Cheyenne Mountain Complex, W.S. Eberhardt, had simply put in the wrong tape. Broadcasters were furious. “The whole darn [system] won’t work,” one station worker told UPI. “They could’ve been dropping H-bombs on us.”

The public was angry, too. It was clear that the nation’s first-line communications system wasn’t as robust as it seemed. “Could similar ‘human error’—here or in the Soviet Union—send American or Soviet weapons into action?” asked The New York Times, slamming “incompetence and unpreparedness at every link of this vital chain.”

In response to the uproar, officials changed the way tests were conducted. Test language was changed, though radio stations could (and sometimes did) transmit it in any way they chose, including in song form. And in 1997, the system was upgraded again into the Emergency Alert System. On October 2, 2018, the government conducted its first test of a "Presidential Alert" sent to all cell phones nation-wide.

The system may be newer, but there’s still one lasting legacy of the disastrous false alarm of 1971: the jarring, screeching sounds you hear during Emergency Alert System broadcast tests. Like the sounds of information being transmitted over a modem, these tones transmit data to broadcasters—data that tells them what kind of situation is in progress and whether the transmission is a test or a false alarm. If the world hadn’t braced for an unknown terror in 1971, those failsafes might never have been implemented.