The attackers opened fire and detonated explosives on the campus. Quickly, the lights went out in Samin's classroom.
"It was very dark, (and) everyone was running. Everyone started screaming," said Samin, who is a US citizen."
It was the scariest moment in my life. I was just thinking about my son and daughter who are in the United States."
Amid black smoke, he took off running with the students and other faculty, and "the smoke entered my mouth" as he fled, he said.
Details of the attack remain unclear but one guard was killed and 21 students were injured, said the head of Kabul Hospitals.
A police operation was set to begin to search for any attackers on the property, according to Interior Ministry spokesman Siddiq Siddiqui.
A senior State Department official told CNN people are trapped inside and insurgents are still on campus. An Afghan response unit is on scene.
There are not a lot of Americans studying at the university, the official said, but there are a number of Americans on the faculty who could be trapped inside.
The US Embassy in Kabul is trying to account for those Americans.