This story was updated April 9 at 6 p.m. ET

Twenty people were injured Wednesday morning in a stabbing at a high school near Pittsburgh. A male suspect, reported to be in 10th grade, is in custody. Police have no known motive as of this time.

Twi victims from Franklin Regional High School in Murrysville, Pa., remain in critical condition, according to officials. A doctor at Forbes Regional Hospital, which was reportedly treating seven students, said the students had critical injuries, but all are expected to survive.

The victims range from age 15 to 60 years old, and most of their wounds were to the lower abdomen. The majority of the students were hurt as the suspect ran down a hallway flashing two large, 8 to 10-inch knives, officials say.

The attack was stopped when he was tackled by the school's assistant principal Sam King. "I'd expect nothing less from him....It feels good to know my dad saved the day," his son said.

Ever since elementary school I knew Mr. King was my favorite. Ηe proved true to that today as a #hero

— Erin Palmer (@acute_erin) April 9, 2014

Students who witnessed the stabbing described a chaotic and bloody scene.

"It was really fast. It felt like he hit me with a wet rag because I felt the blood splash on my face. It spurted up on my forehead," victim Nate Moore, 15, told the AP. The attacker "had the same expression on his face that he has every day, which was the freakiest part," Moore said. "He wasn't saying anything. He didn't have any anger on his face. It was just a blank expression."

"I was walking into the school and a stampede of people were running after me," a student who witnessed the stabbing told NBC News. "They were screaming, 'Go to your cars! Go to your cars! Someone is stabbing people!'"

“In a situation like this, you want the students to run,” Murrysville Police Chief Thomas Seefeld said, noting that they did the right thing.

One alleged victim of the attack posted a selfie to Instagram, showing a wound on his right arm.

“I was shocked and saddened upon learning of the events that occurred this morning as students arrived at Franklin Regional High School. As a parent and grandparent, I can think of nothing more distressing than senseless violence against children. My heart and prayers go out to all the victims and their families,” Pennsylvania Governor Tom Corbett said in a statement.

Three helicopters and dozens of ambulances had been dispatched to Franklin Regional High School, CBS News reported, after the stabbing occurred in hallways and multiple classrooms in the building’s science wing shortly before 7:30 a.m. ET.

The school issued this emergency bulletin on its website: “A critical incident has occurred at the high school. All elementary schools are canceled, the middle school and high school students are secure.”

This is the first major stabbing in the U.S. this year. In Texas last year, at least 14 people were injured in a stabbing spree at Lone Star College outside Houston in April. In September, one student was killed and three were injured in a stabbing at Spring High School, also near Houston.

Stabbings are much more common abroad. In March alone, 29 people were killed and 130 wounded in a knife attack at a railway station in southeastern China.