Hossaini, who is studying political science at the university, said he was in his classroom when he heard an explosion on the southern side of the compound.
"I asked my classmates to lie on the ground, and I ran to the other side, to the south side, to see what is going on," Hossaini told ABC News. Hossaini said when he looked out the window he saw one of the attackers, who he said aimed a weapon and shot at him.
Hossaini described the harrowing moment when the students realized they could not safely escape, and they were forced to barricade themselves into the classroom by pushing chairs and desks against the door.
"We didn't know which side we should run away, otherwise we could just jump," Hossaini said, "but because I saw that guy, the terrorist walking on the south side of that building, I just realized that we have to stay."
Hossaini said they could hear the attacker in the building going to the classrooms "one by one."
"He kicked our door by his feet two times. So at this time, everybody panicked and they went to the window," Hossaini said, saying that then he heard an explosion and nearly fell from the window.
Then he decided to jump. Hossaini said that when he was on the ground, he ran toward the emergency exit, passing a dead body on the way. Hossaini said eventually security saw them and helped them escape.
Hossaini said he wounded both of his hands on broken glass, but added that some of his classmates' injuries were far more serious.
He said that he is still trying to track down some of his friends and he didn't know how many attackers there were, besides the one that kicked on the door of his classroom.
He also said he was unsure of what kind of weapon the attacker carried, "but it was like a Kalashnikov."
Just as news of the attack broke, Hossaini tweeted, “Help we are stuck inside AUAF and shooting flollowed [sic] by Explo this maybe my last tweets.”
The tweet, which was shared hundreds of times before Hossani removed it, painted a dire portrait to outsiders of the attack that was going on inside of the school.
Hossaini won a Pulitzer Prize in 2012 while working with Agence France-Presse "for his heartbreaking image of a girl crying in fear after a suicide bomber's attack at a crowded shrine in Kabul," according to the Pulitzer website.