Soofi, 34, and Elton Simpson were shot dead by police on Sunday as they tried to storm the controversial cartoon drawing event.
The Islamic State group has claimed responsibility, although US officials caution it is too early to draw a firm link.
Soofi studied at the USD 20,000-a-year International School of Islamabad from 1992 to 1998, where contemporaries said he was funny, popular and charming and showed no inclination towards extremism.
"Whatever fundo [fundamentalist] indoctrination occurred, [it] happened after he graduated and moved to USA -- here he was simply a cool kid with a bright future," the contemporary said.
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Soofi's mother taught art at the heavily-guarded school, which is popular with diplomats and rich Pakistanis, several of his contemporaries said.
He took part in theatre productions, and another schoolmate said Soofi starred as the lead in "Bye Bye Birdie", a musical inspired by Elvis Presley, transforming him from a sweet, shy boy into "a confident heartthrob".
She added she was shocked by news of what Soofi had done in Texas.
"I had noticed on Facebook that he had become Islamic but never thought to such extremes," she said.
Another former pupil who was a close friend of Soofi's brother said past students were "traumatised" by events in Texas.
"It's a very small community and everyone stays in touch. It's really sad to hear," he said.
The former pupil said Soofi's family were "well off" and lived in a good house but paid no fees at the Islamabad school as his mother taught there.
The school refused to comment on the case and security guards stopped journalists from approaching today.