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Two students were killed Tuesday and 18 other people were wounded when a 15-year-old boy armed with a handgun opened fire inside a Kentucky high school, authorities said.

Terrified students ditched their backpacks and scrambled to get away, and within minutes of the shots' having been fired, sheriff's deputies were at Marshall County High School in Benton, where they disarmed the student and took him into custody, officials said.

But it was too late to save Bailey Nicole Holt, 15, who died at the scene, and Preston Ryan Cope, 15, who died later at a trauma center, Kentucky State Police Commissioner Richard Sanders said early Tuesday night.

Sixteen of the wounded were injured by gunfire and the four others were hurt while trying to escape, state police said Tuesday night, revising Sanders' earlier report that 14 people were shot. Three of the victims were listed in critical condition Tuesday night at Vanderbilt University Medical Center in Nashville, Tennessee.

"In addition to those family members that have lost loved ones, that have had loved ones injured or hurt or traumatized, we pray for these people," Sanders said.

Authorities declined to identify the suspect or discuss a possible motive, but Sanders said he would likely face two counts of murder and "numerous" counts of attempted murder.The FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives also joined the investigation.

Bailey Holt, one of the victims of the Marshall County High School shooting in Kentucky.

The small town was in shock as teachers and counselors tried to calm terrified students who described the chaos in the classrooms when the bullets started flying at 7:57 a.m. (8:57 a.m. ET), shortly after the shooter entered the school's common area, according to Sanders.

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Two minutes later, dispatchers got the first 911 call, he said, and police were on scene by 8:06 a.m. The suspect was taken into custody almost immediately by the first officer on the scene, he said.

"They were busting down the gates and fences just to get out," Shea Thompson, whose teenage siblings were inside the school when the shooting started, told NBC News.

Greg Rodgers, 17, a junior, said that when he arrived at school, he saw students racing out of the building.

"I pulled off to the side of the road because everyone was running to the main road," Greg said. "I asked my friend what was going on, and he told me that there was a school shooting. I was shocked. He said that someone had just shot up the school."

Greg said the suspect opened fire as students were heading to classes.

"I'm distraught from all of it. I couldn't really focus driving home," he said. "I was shaking a lot driving back to my house. I'm still shaking."

Thompson said her 15-year-old brother, Shawn, a freshman, called her from the school in "a complete panic" around 8:02 a.m.

"He was yelling: 'Someone's shooting! Someone's shooting!," said Thompson, 26, of Benton.

Thompson said her 16-year-old sister, Kristin, a sophomore, was in the common area near the cafeteria. She said Kristin told her that a bullet pierced her friend's backpack but that the friend wasn't hurt.

Clyde Lee was at North Marshall Middle School waiting to collect Isaac Robinson, his nephew, a sophomore who was being bused there from the high school. He said he had just learned that a young girl they know was badly wounded.

"There are a lot of parents crying," Lee said. "It's a very bad situation here. We had a family friend who was shot in the chest. We don't have any other information on her condition."

Emergency crews respond to Marshall County High School in Benton, Kentucky, after a fatal school shooting Tuesday. Ryan Hermens / The Paducah Sun via AP

Lee said he never imagined "something like this happening in such a small town" like Benton.

"No one should have to go through something like this," he said.

Benton is a city of about 4,300 residents in western Kentucky, about 120 miles northwest of Nashville. It is also about 40 miles southeast of West Paducah, Kentucky, where a 14-year-old student at Heath High School opened fired in 1997, killing three people and wounding five others as they were praying.

The shooting Tuesday came a day after a 15-year-old girl was wounded at Italy High School in Texas after someone opened fire inside the cafeteria. No motive was immediately given.

"Our nation's schools should be some of the safest spaces in our communities," said Gabby Giffords, the former congresswoman-turned-activist, who noted that there have been 13 mass shootings already this year. "And it's only January."

CORRECTION (Jan. 24, 7:20 pm ET): An earlier version of this article misstated the location of Vanderbilt University Medical Center. It is in Nashville, Tennessee, not Knoxville.