The balcony in Kabul where the head of al-Qaida was killed was a spot Dan Smock knew well. It used to be his – when he worked in Afghanistan on a US government aid project – and the views were spectacular.

Smock enjoyed starting the day looking out at the Afghan capital, as did the world’s most wanted terrorist, from the villa they both called home, several years apart.

“Reports said the CIA had intelligence that he liked to stand on the balcony, and I thought, ‘Of course he would, it was a nice balcony,’” Smock said in a phone interview.

“When the Kabul smog lifts you can see the mountains in the morning, and it’s next to an open field,” he said. He put up bamboo matting as a privacy screen, which was still there when a US drone struck Ayman al-Zawahiri down, so the terrace was not overlooked.

“It felt like you could hang out there without anyone noticing who it is, unless someone was really paying attention. And clearly (this year) someone was.”

The cream house, with sandy-orange detailing and green-mirrored balcony walls was in a neighbourhood famous for land grabs by the warlords and technocrat elite of the Afghan republic, which collapsed last summer.

As the war escalated, many of the villas they crammed into small plots of land were rented by the NGOs and contractors, such as Smock’s employer.

Smock’s old home had a distinctive external lattice feature between the floors that he first noticed in photographs posted on social media at the weekend when it was hit by a suspected US drone strike. He was a little surprise and disconcerted to see the windows smashed.

“When I saw it I thought ‘that’s my old house’,” he said. “These villas are garish as all hell but unique and this one especially, it was built on such a narrow footprint.”

Then, on Monday evening, the US president, Joe Biden, told Americans that the al-Qaida leader, Zawahiri, had been the target.

And Smock, a US military veteran of the war in Iraq, who also spent years working as a civilian in Afghanistan, realised he had lived in the same space as one of the men who plotted the 9/11 attacks.

“It’s an incredibly surreal thing. Things change, and things change quickly, but at that level? That’s a little intense. You’ve got public enemy number one, with a $25m bounty on his head, literally living in the same space you lived in previously,” he said.

“I keep running through the reality of him being in the same rooms I was in.”

The CIA created a detailed model of the house, US media reported, to help understand how a strike might affect the structure, and whether Zawahiri could be killed without harming others.

The reason the area appealed to US government contractors is probably the same reason it was seen as a good place to host the al-Qaida leader. It is essentially a quiet, closed-off neighbourhood near the seat of power.

“Down by the [Ghazanfar] bank and Spinneys [supermarket], there are two entrances on either side. If you control those you control the whole neighbourhood,” said Smock.

He described a tall, relatively narrow house, set back from the security wall behind a paved garden area lined with shrubs. The main doors opened on to a staircase that ran up through the centre of the house, with strange acoustics.

“If you said anything on the ground floor it echoed up all the floors. It was like living in a speaker box, even if you were not speaking loudly.” Smock moved in with about half a dozen colleagues – for security reasons foreigners took jobs without families and were regularly put up in shared houses.

At the time there was a kitchen on the ground floor, three bedrooms on the higher floors and on the top a small apartment space, with a living room and en suite bathroom. Opposite it was the door on to the balcony where Zawahiri was killed.

Biden hailed the drone strike as a counter-terrorism triumph, but to Smock the fact that Zawahiri had been there at all underlined how terribly Washington and its allies had failed in Afghanistan.

After billions of dollars spent, and years of promises to improve the lives of Afghans while making the US safe, Afghan girls are barred from high school, the economy is collapsing and al-Qaida’s head ran his operation from the heart of the capital.

“[The western mission] failed so spectacularly that the people who took over in Kabul could do an Airbnb for the al-Qaida CEO in a house that had been run by USAid contracting dollars for a decade plus,” Smock said.

“It made me very sad. The news brought me the full weight of understanding. After all those efforts, the rock has fully rolled down the hill.”