JAN 20 2016, 2:24 PM ET Gunmen Storm Pakistan University, Shoot at Least 80: Official

CHARSADDA, Pakistan — Militants using heavy fog as cover stormed a Pakistani university early Wednesday, killing some 20 people and injuring around 60 others, many of them critically.

Yelling "Allahu Akbar" ("God is great") and brandishing AK-47 rifles, the quartet of gunmen struck classrooms and dorms at the Bacha Khan University in Charsadda, killing some of the victims execution-style with a shot to the head, police and witnesses said.

"There was heavy fog in the area when armed men entered ... and started firing at the security guards, students and teachers" regional police chief Saeed Wazir told NBC News.

Wazir said the attack appeared to be well-planned because "they knew where to go and they knew who to go for."

All four were wearing explosives vests but only one of them was able to detonate his deadly payload, police said. The rest died in a fierce gun battled with Pakistani military and at least two appeared to be teenagers.

"Within 45 minutes forces were active and the attackers contained," military spokesman Lt. Gen. Asim Salim Bajwa said.

In the aftermath, NBC News reporters toured the men's dormitory and saw blood of the floors and the walls pitted by bullets. Much of the two-story building was destroyed.