BLACKSBURG — They are the images and descriptions that cannot help but stick.

Loud bangs that sounded like construction work. Wordless gunfire. Students and professors cowered in classrooms, people blockaded doors. Bloodstained clothing.

Substitute teacher Haiyan Cheng walked into her Issues in Scientific Computing class at 9 a.m. Monday geared to lecture about analytical solutions to senior math and computer science majors.

Forty minutes in, she heard it through the open door of her classroom.

“They were really loud. I was not sure where it came from, but it seemed very close to us,” she wrote in an e-mail. “We all thought it was some kind of construction.”

After a 10-second pause, Cheng was ready to begin a new topic.

Again. The sounds.

Cheng and a female student sitting next to the door popped their heads outside to investigate and heard the noises coming from Room 208, down the hall to the left, not a construction site.

About two seconds later, a man walked out of Room 208. Cheng said she didn’t keep her head out too long, but just enough to notice the first disturbing image — a handgun.

She “came back in immediately, the gun shot right besides our ear,” she wrote. “My ear is hurting by the loud sound.”

She ordered her 11 students to duck down. Four men dragged a table, which stood next to the podium where she had just been lecturing, to block the door.

Students crouched behind the podium, in the back of the room, all the while hearing shooting from the hallway. “The gun shot keeps going, 'Bang, bang, bang,’ all I can do is knee down there and pray to God 'Please stop him, please stop the gun fire, please,’” Cheng said.

Then, shooter Cho Seung-Hui moved toward Cheng’s class.

He tried to open the barricaded door, but students pushed hard, keeping him at bay.

“Then, he began to shoot the door,” Cheng wrote. “The bullets came through the door, woodchips and metal pieces are everywhere. One bullet even hit the podium.”

Meanwhile, a similar scene played out in Norris 204, professor Liviu Librescu’s class.

Students in that solid mechanics class were viewing slides when they heard gunshots in room 206. “At first, everybody was kind of stunned and confused. We didn’t really do anything,” said junior Richard Mallalieu.

But, like Cheng’s class, when they realized what was happening, they shielded themselves with desks and chairs. One student searched for a way out in the front of the class, but when he heard the gunshots coming closer, retreated, Mallalieu said.

Someone suggested jumping from the windows. The old, metal-frame panes were easy to open, so students leaped, some hanging from the 20-foot ledge, Mallalieu recalled Tuesday.

Although some suffered broken limbs from the fall, about 15 of the 20 students survived the shootings. They headed for Patton Hall, where they were holed up for about an hour.

Cheng’s students decided jumping from the second-floor video room was a bad idea. They didn’t know what was outside, she said.

While they waited, more gunshots. About 40, but some farther and farther away, she wrote.

“It was terrible. We waited and waited. We hard several unclear shouts from outside, 'show me your hands, show me your hands [sic] ...’ but we are not sure if that they were police or not,” she wrote.

Finally, someone knocked on the door. Leery, students went to the door. A police officer, gun drawn, told Cheng and her students to put their hands up and go to the rear of the classroom, she wrote.

Then, after checking that no one had been injured, Cheng and her class followed the officer, single file, out of Norris Hall.

Cheng remembers seeing a gun clip in the hallway directly outside her classroom.

“We try not to step on the blood and run quick,” she wrote. “I didn’t even look at the other classroom.”

Students behind her began crying, but Cheng clasped her hands and ran toward safety in Randolph Hall.

Not everyone reached that point.

Librescu, and 30 others, died.

“I don’t consider myself a victim in this,” Mallalieu said.

Staff writer Angela Manese-Lee contributed to this report.