The weather in the northeastern United States on the morning of Tuesday, September 11, 2001, was what air traffic controllers describe as “severe clear.” A high-pressure system had blown the previous day’s storms out to sea; skies were an intense cobalt blue. “A severe clear day means that you're going to have a great day in air traffic control,” says Michael McCormick, who oversaw all air traffic in the northeastern United States that day out of the Federal Aviation Administration’s New York Center in Ronkonkoma.

It was not a great day in air traffic control.

As the morning progressed, four separate terror attacks unfolded in the skies, with hijackers using commercial aircraft as weapons. Perpetrators deliberately flew three of those planes into iconic buildings in New York City and Washington, D.C., while a fourth crashed in a Pennsylvania field before reaching its target. Through shock and confusion, aviation professionals tasked with keeping America’s skies safe had to stay cool and make unthinkable decisions—to call in fighter jets, shut down U.S. airspace and land all airborne planes—in response to dire, unprecedented events. “This [was] a dynamic event with details changing from moment to moment, from second to second,” McCormick told HISTORY.

Here’s a detailed look at how several key U.S. air traffic control officials experienced their profession’s most catastrophic day:

In the windowless Nashua, New Hampshire bunker that houses Boston Center—the FAA facility that guides aircraft crisscrossing the skies above New England and much of New York—Peter Zalewski, a 20-year veteran, starts his shift at 7 a.m. Soon, he is trying to figure out what is happening with American Airlines Flight 11. The plane had taken off from Boston’s Logan Airport at 7:59 a.m. After guiding the flight’s initial climb, Zalewski can’t get a response from the pilots.

8:13 a.m.

Boston Center/Zalewski: American 11, climb, maintain flight level three-five-zero.

Boston Center/Zalewski: American 11, Boston?

Boston Center/Zalewski: American one-one, the American on this frequency, how do you read me?

“I’m like, my god, maybe they’re drinking Dunkin’ Donuts coffee up there,” Zalewski recalls thinking. Controllers chalk the lack of response up to multitasking. No one is worried—yet.

8:14 a.m. United Airlines Flight 175 takes off from Boston’s Logan Airport.

8:20 a.m. American Airlines Flight 77 leaves Dulles Airport, heading for Los Angeles.

8:21 a.m. The hijackers turn off the transponders for AA 11. Then controllers hear Mohamed Atta when the lead hijacker accidentally broadcasts a message meant for passengers on the air traffic control channel.

8:24 a.m.

Atta: We have some planes. Just stay quiet and we’ll be OK. We are returning to the airport.

Zalewski—who often speaks to pilots from Saudi Arabia and Egypt—recognizes Atta’s accent as Middle Eastern.

8:33 a.m.

Atta: Don’t try to make any stupid moves.

At this point, controllers reach out to their military contacts.

8:37 a.m.

Boston Center/Joseph Cooper: We need you guys to scramble some F-16s or something to help us out.

Northeast Air Defense Sector (NEADS)/Sgt. Jeremy Powell: Is this real-world or exercise?

Boston Center/Cooper: This is not an exercise, not a test.

Controllers note that AA 11 has deviated from its flight path. Bruce Barnett, operations manager at New York Center in Ronkonkoma, warns McCormick that a possibly hijacked flight is heading toward New York at full speed.

8:41 a.m.

UAL 175 to New York Center: We heard a suspicious transmission on our departure from BOS sounds like someone keyed the mike and said everyone stay in your seats.

Controllers fail to pass that information along; they’re soon too busy trying to keep track of multiple hijacked planes.

8:42 a.m. (approx.) Hijackers seize control of UAL 175.

8:42 a.m. United Airlines Flight 93 takes off from Newark Airport, bound for San Francisco.

8:44 a.m.

Madeline Sweeney (flight attendant on AA 11): I see water. I see buildings… We’re flying way too low.

8:46 a.m. AA 11 crashes into the North Tower of the World Trade Center in New York. Controllers initially think it’s a small Cessna flying under visual flight rules, and not part of their responsibility.

8:46 a.m. Dave Bottiglia in New York Center sees the radar blip for AA 11 disappear. “Well, we know he’s not high altitude anymore,” he comments. Concerned, he pings another plane.

8:51 a.m.

New York Center/Bottiglia: “United, do you read New York?”

There is no reply.

8:51 a.m. Air traffic controllers in Indianapolis receive the last routine communication from AA 77.

8:52 a.m.

Bottiglia notes that UAL 175 has changed its transponder and is climbing rapidly. “We may have a hijack,” he tells another controller. Mike McCormick realizes UAL 175 probably “was going to be another weapon to be used on the World Trade Center.”

8:54 a.m. AA 77 deviates from its course and pulls its transponder. Could it have crashed? Indianapolis controllers consider the possibility.

About 9 a.m. Gerald Earwood, piloting a Midwest Express flight from Milwaukee to New York’s La Guardia Airport, can’t reach controllers. “There was no chatter, no talk, no anything,” he later told Garrett Graff, who compiled an oral history of the day’s events in the book The Only Plane in the Sky. Suddenly, a controller shouts an order for him to make an immediate turn to avoid colliding with UAL 175. “I have never had a controller scream like that.”

9 a.m. Controllers struggle to keep up with unfolding events. There was “some slowness in realizing that we were dealing with an additional hijack, because, by gosh, we already had two,” recalls Dan Creedon, departure controller at Reagan National Airport.

9:01 a.m.

New York Center/Peter Mulligan to FAA Command Center: We have several situations going on here. It is escalating big, big time…”

9:02 a.m. New York Center controllers try to get a visual fix on UAL 175. The “severe clear” weather means staffers at the New York Terminal Radar Approach Center (TRACON) see it dive toward the World Trade Center.

9:03 a.m. UAL 175 flies into the South Tower of the World Trade Center.

TRACON to New York Center: “Another one just hit it hard… The whole building just, ah, came apart.”

9:04 a.m. Ben Sliney, in his first day on the job as national operations manager for the FAA in Herndon, Virginia, orders a “ground stop,” banning planes nationwide from taking off.

9:05 a.m. Mike McCormick orders “ATC Zero,” or the complete shutdown of airspace around New York City; no planes can take off or land. “It's the first time that ATC Zero had ever been used for an event like this,” he tells HISTORY. “It was intended to be utilized when you can no longer provide air traffic control services generally due to equipment malfunctions.”

9:10 a.m. Terry Sliney tells his staff to collect any and all reports of suspicious activities involving planes in the air. They post details of about two dozen flights to a dry erase board in the middle of the room.

9:10 a.m. AA 77’s radar blip reappears; it’s now in air space managed from Washington, D.C.

9:10 a.m. In Cleveland, air traffic manager Rick Kettell recognizes similarities between the hijacked planes. All were transcontinental flights “obviously full of fuel.” Kettell tells his team to look out for comparable flights.

9:19 a.m. Ed Ballinger, a United Airlines dispatcher, warns planes he is following (including UAL 93) about possible hijackers.

9:20 a.m. Indianapolis controllers—now aware of the World Trade Center events—begin to suspect that AA 77 has been hijacked.

9:24 a.m. UAL 93 checks in with Cleveland Center.

UAL 93/Jason Dahl: “Good morning Cleveland, United 93 with you at three-five-oh [35,000 feet].

9:27 a.m. John Werth, the Cleveland controller in charge of UAL 93, makes the last regular contact with the plane’s crew.

9:28 a.m. Werth hears sounds coming from one of his aircraft; he can’t immediately tell which one. “It sounded like a life-or-death struggle. It was some screaming and some guttural sounds.”

9:30 a.m. Langley Air Force Base fighters are directed to the Baltimore area to intercept AA 11, which is believed to be heading toward Washington.

9:32 a.m. Controllers at both Dulles and National airports spot an incoming jet, flying rapidly and far too low. Creedon’s boss calls the Secret Service to tell them to evacuate the White House—ASAP.

Creedon: “He's like, 'we got an untracked target, we don't know who he is' and he said 'eight miles away from the White House, heading right toward it.'

9:32 a.m. Cleveland controllers hear a transmission coming over the air, apparently intended for passengers.

Ziad Jarrah: "Keep remaining sitting. We have a bomb on board."

9:36 a.m. UAL 93 starts to climb; pilots don’t respond to Werth. The pattern is familiar.

9:37 a.m. AA 77 crashes into the western side of Pentagon

Creedon: “He hit the Pentagon at, you know, I think like 560 knots (about 640 miles per hour), which is an insane speed.”

9:39 a.m. Cleveland controllers and nearby pilots overhear a transmission from UAL 93.

Jarrah: Uh, is the captain. Would like you all to remain seated. There is a bomb on board and are going back to the airport. Please remain quiet.”

9:41 a.m. The hijackers pull the transponder from UAL 93 and make a 120-degree turn. The plane is now heading for Washington, D.C.

9:42 a.m. For the first time in aviation history, the FAA orders all planes within U.S. airspace to land immediately and closes airspace.

9:45 a.m. Terry Biggio begins overseeing the landing process in the Northeast. “We told them, you’re not leaving our airspace. Pick an airport.” Sliney says 700 of the 4,000 planes in the air landed within 10 minutes.

10 a.m. UAL 93 passengers vote to retake the aircraft. The cockpit voice recorder picks up details of the struggle that follows.

10:03 a.m. Cleveland Center tracks the plane all the way to Shanksville, Pennsylvania, and monitors—on radar—its final crash into the ground. Stacey Taylor Parham, one of the controllers, asks a nearby corporate jet to look for smoke.

10:07 a.m. The pilots report seeing a “big black hole, and it was smoking.”

10:15 a.m. By this time, the worst of their day in the lives of these air traffic controllers was over—and the post mortem had begun.