| Students comfort each other at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, where a student shot at least one other student at a Colorado high school Friday before he apparently killed himself, authorities said. The shooter entered Arapahoe High School in a Denver suburb armed with a shotgun and looking for a teacher he identified by name, said Arapahoe County Sheriff Grayson Robinson | 
     CENTENNIAL, Colo.
     (AP) -- A teenager who may have had a grudge against a teacher 
opened fire Friday with a shotgun at a suburban Denver high school, 
wounding two fellow students before killing himself.
Quick-thinking
 students alerted the targeted teacher, who quickly left the building, 
and police immediately locked down the scene on the eve of the Newtown 
massacre anniversary, a somber reminder of how commonplace school 
violence has become.
One of the wounded 
students, a girl, was hospitalized in serious condition. The other 
student suffered minor gunshot-related injuries and was expected to be 
released from the hospital Friday evening, authorities said.
A third person was being treated for unspecified injuries but had not been shot, a hospital spokeswoman said.
Arapahoe
 County Sheriff Grayson Robinson initially reported that the most 
seriously hurt student was wounded after confronting the gunman, but he 
later said that did not appear to be the case.
The
 gunman made no attempt to hide the weapon when he entered the school 
from a parking lot and started asking for the teacher by name, Robinson 
said.
When the teacher learned that he was 
being targeted, he left "in an effort to try to encourage the shooter to
 also leave the school," the sheriff said. "That was a very wise 
tactical decision."
Jessica Girard was in math class when she heard three shots.
"Then
 there was a bunch of yelling, and then I think one of the people who 
had been shot was yelling in the hallway `Make it stop,'" she said.
A
 suspected Molotov cocktail was also found inside the high school, the 
sheriff said. The bomb squad was investigating the device.
Within 20 minutes of the first report of a gunman, officers found the suspect's body inside the school, Robinson said.
Several
 other Denver-area school districts went into lockdown as reports of the
 shooting spread. Police as far away as Fort Collins, about a two-hour 
drive north, stepped up school security.
Arapahoe
 High students were seen walking toward the school's running track with 
their hands in the air, and television footage showed students being 
patted down. Robinson said deputies wanted to make sure there were no 
other conspirators. Authorities later concluded that the gunman had 
acted alone.
Nearby neighborhoods were jammed 
with cars as parents sought out their children. Some parents stood in 
long lines at a church. One young girl who was barefoot embraced her 
parents, and the family began to cry.
The 
shooting came a day before the anniversary of the Newtown, Conn., attack
 in which a gunman killed 20 children and six adults at Sandy Hook 
Elementary School.
Arapahoe High stands just 8
 miles east of Columbine High School in Littleton, where two teenage 
shooters killed 12 classmates and a teacher before killing themselves in
 1999. The practice of sending law enforcement directly into an active 
shooting, as was done Friday, was a tactic that developed in response to
 the Columbine shooting.
Since Columbine, 
Colorado has endured other mass shootings, including the killing of 12 
people in a movie theater in nearby Aurora in 2012. But it was not until
 after the Newtown massacre that state lawmakers moved to enact stricter
 gun-control laws. Two Democratic lawmakers were recalled from office 
earlier this year for backing the laws, and a third recently resigned to
 avoid a recall election.
The district 
attorney prosecuting the theater shooting, George Brauchler, lives near 
the high school. At a news conference, he urged anyone who needed help 
to call a counseling service and gave out a phone number.
Tracy
 Monroe, who had step-siblings who attended Columbine, was standing 
outside Arapahoe High on Friday looking at her phone, reading text 
messages from her 15-year-old daughter inside.
Monroe
 said she got the first text from her daughter, sophomore Jade Stanton, 
at 12:41 p.m. The text read, "There's sirens. It's real. I love you."
A
 few minutes later, Jade texted "shots were fired in our school." Monroe
 rushed to the school and was relieved when Jade texted that a police 
officer entered her classroom and she was safe.
Monroe was family friends with a teacher killed in the Columbine shooting, Dave Sanders.
"We didn't think it could happen in Colorado then, either," Monroe said.
After
 hearing three shots, freshman Colton Powers said everyone "ran to the 
corner of the room and turned off the lights and locked the door and 
just waited, hoped for the best. A lot of people, I couldn't see, but 
they were crying. I was scared. I didn't know what to do."
His mother, Shelly Powers, said she first got word of the shooting in the middle of a conference call at work.
"I dropped all my devices, got my keys and got in my car," she said. "I was crying all the way here."
More
 than 2,100 students attend Arapahoe High, where nine out of 10 
graduates go on to college, according to the Littleton Public Schools 
website.