From his desk at the FBI's Washington Field Office, Pete Lapp could see smoke blackening the sky above the Pentagon.

After the twin towers fell, a sight he and other agents had watched on a television in a supervisor's office, Lapp assumed they would get sent to New York to aid in the investigation. He called his mother to ask if she could go to his house and help his wife with their two-week-old baby. He didn't know how long he would be gone.

Then, he saw the smoke.

“And I knew,” he recalls. “I knew we were not going to New York. We were going to the Pentagon.”

The Field Office is not among the unmarked, undisclosed government buildings in the DC area. An FBI flag waves outside. So, on that day, there was reason to believe the building could also end up a target. As Lapp tells it, the staff was ordered to gather in the basement. They clustered there for about 20 minutes, waiting for the unknown, and then collectively decided to leave. There was work to be done.

Lapp was close to making an arrest in a high-profile espionage case, the biggest of his career, but on that day, he was tasked with trying to find witnesses to the Pentagon attack. The Government needed to confirm that the damage was caused by a plane and not a bomb. He and a small group went to the Sheraton, and there, they found a woman who had seen the plane from her hotel window.

“She was obviously a mess,” Lapp recalls.

The way he says it leaves a sense that she might in some ways still be a mess. That he might in some ways still be a mess. That many people in the Washington region might in some ways still be a mess. The 51-year-old holds back tears as he recalls the specifics of that day. Twenty years have passed but his memories remain sharp-edged.

“Life changed for ever after that day for me as a person and for me as an FBI agent,” he says. “That day still haunts me. I remember going to work every day afterward and listening to [radio station] WTOP and listening to the wall-to-wall coverage and listening to the body count, and crying.”

He also remembers what happened afterward.

“We couldn't catch a break here in this region,” he says.

The 9/11 attacks shook the nation, but in the Washington region, that devastating day was the start of 13 months of terror. After it, letters filled with deadly anthrax began arriving in mailrooms, and then the DC Sniper attacks left people fearing a bullet could find them anywhere at any moment. People started ducking at petrol stations, parking as close as possible to store entrances and envisioning a gun aimed at them, even when it wasn’t.

“While I’m walking from my apartment to the Metro Station, am I going to get shot? Am I going to get shot getting off the train?” Terry Cooper recalls thinking at the time. “It was just so random. You could just be walking along minding your own business and then boom, you got one in between the eyes and you were no longer here.”

Every year around this time, she says, she starts to get flashbacks of those 13 months.

She worked for Fema [the Federal Emergency Management Agency] at the time, and soon after the 9/11 attacks, she found herself working 12-hour shifts, seven days a week. She had been trained to deal with survivors of tornadoes and other natural disasters, she says, but at that time she was taking calls from people who had lost jobs, homes and loved ones in the attacks.

“I took two suicide calls on my first day back,” she says. “What do you say to someone who is suicidal when you’re not trained to do that? The last thing I wanted to hear was someone jumping out a window or pulling a trigger.”

She recalls grabbing a blank sheet of paper, using a Sharpie to write “suicide” and holding it up for someone to see. That prompted a supervisor to come over, copy the address from her screen and call the police.

On another day, as a writer for a newsletter that Fema published, Cooper went to capture the scene at a makeshift memorial that had formed in between the Pentagon and Arlington Cemetery. It took her three hours to get through it.

“I literally dropped my camera and my notepad and everything, and fell to my knees and started to cry,” she recalls. “Just to see that much pain concentrated in one area, to look to my right and see the crash site and to look straight ahead and see Arlington Cemetery, it was too much.”

The terrorist attacks left nearly 3,000 people dead in New York, Pennsylvania and northern Virginia. Of those, 184 people were killed at the Pentagon, and many more were injured.

But those numbers were still uncertain when the anthrax attacks started creating a separate death toll and spreading a different type of fear. The FBI would name a Fort Detrick scientist who killed himself as responsible for the mailings, but not before five people died and 17 were admitted to hospital.

The first letters containing the white powder were postmarked September 18, 2001. Another was dated November 9, 2001.

After the November letter was sent, the FBI released photographs of all three envelopes and asked for the public’s help in identifying who might be responsible. The agency’s public plea pointed out the unique characteristics of the writing: how dashes were used to connote dates instead of slashes. How the letters were all upper case. How the words “can not” appeared instead of “cannot”.

Cooper, at the time, was working in an undisclosed building in an office near the mail room. But for a short while her work took her to another floor, and that’s where she witnessed Secret Service agents block off a hallway. Someone had found white powder in the women’s bathroom.

It turned out to be nothing dangerous, but during that time, every out-of-place white speck was suspect. Sugar. Flour. Baby powder.

Between October 1 and 16, the FBI responded to more than 2,300 incidents involving suspected anthrax or other dangerous substances, according to a statement released at the time. The FBI also dedicated agents to the Amerithrax Task Force, which conducted thousands of interviews and executed dozens of search warrants across six continents.

Lapp was not part of that task force. He was working on a different high-stakes case. When the first anthrax-filled letter was sent, he and a fellow agent were days away from arresting Ana Montes, whom The Washington Post would later describe as “the most important spy you’ve never heard of”.

While working as a US intelligence analyst, Montes had spied for Cuba for 17 years. That there wasn’t much news coverage of her arrest shows just how many big stories were breaking at once. Lapp recalls sitting in bed, holding his infant son on a Sunday, and seeing CNN’s chyron announce the arrest. Then it went away, and didn’t appear again.

For a while, it seemed that the region could exhale, but then came the bullets. A sniper was shooting people in Maryland, Virginia and DC, randomly. At night. During the day. At petrol stations. In front of a Home Depot. The victims were Black and White and Latino. They were kids who barely started their lives and older people who were not yet ready to surrender theirs.

Schools cancelled recess and after-school activities. Petrol stations put their pumps under tarps to shield their customers.

Lapp and Cooper both recall that time as the most frightening for them on a personal level during those 13 months.

They do not know each other. They do not live in the same neighbourhood or work in the same field.

They are just two people, of many, who happen to be linked by a place and a time: the Washington region between September 11, 2001 and October 24, 2002, when authorities arrested the two people responsible for the sniper attacks.

They are just two people, of many, who still remember the memorials for the Pentagon workers who didn’t survive, the way people opened letters far from their faces and that white van authorities mistakenly linked to the sniper attacks.

Authorities would later realise John Allen Muhammad and Lee Boyd Malvo, who killed 15 people across the country, ten of them in the DC area, were actually driving a blue Chevy Caprice.

But before that was known, Lapp and Cooper both spent time thinking about that van. Lapp’s background as a police officer made him want to go search for it. And Cooper’s background as a writer made her wonder every time a white van passed if she should be taking photos of the licence plate and memorising the driver’s face.

Cooper finds writing therapeutic and since retiring, the 58-year-old has spent more time doing it. In a piece she wrote recently, she describes those long-ago months as “a living nightmare”.

She writes: “I was never so glad to see 2001-02 disappear. Not even 2020 outdid those 13 fateful months living in the DC metro area.”

• Theresa Vargas is a local columnist for The Washington Post. Before joining the Post, she worked at Newsday in New York. She has degrees from Stanford University and Columbia University School of Journalism