“I would like to prove to the authorities, my family and society in general that I don’t carry the views I had before my arrest,” he wrote, “and also I can prove that at the time I was immature and now I am much more mature and want to live my life as a good Muslim and also a good citizen of Britain.”

Later that year, he was enrolled in the Healthy Identity Intervention Program, the government’s main vehicle for dealing with people convicted of offenses linked to terrorism. While enrolled in that course, he wrote long reports about his putative progress and repeatedly asked for a chance to prove that he was no longer a threat.

A probation service official familiar with Mr. Khan’s case said that he carried out an elaborate deception of all the agencies that had been monitoring him. The official insisted that nothing in his behavior suggested he would do anything improper, let alone the terrible attack he pulled off.

Before the attack, Mr. Khan was under active surveillance by the MI5 domestic intelligence agency, which had set his level of threat to the public as “low to medium.” He had the highest level of security measures applied to his parole, according to an internal probation report. The probation official said that Mr. Khan would have to have shown significant progress to be allowed to attend the conference.

He displayed those feigned signs of progress on the morning of the conference, when he spoke about his rehabilitation efforts — a “compelling success story,” as one conference participant described it.

But during a break in the conference program, the BBC reported, Mr. Khan disappeared into a bathroom and re-emerged wearing a fake suicide vest with two large knives taped to his hands — it is unclear how he got the weapons into the building.