In this still image taken from video provided by Fox 31 Denver, police respond to reports of a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.
Updated 5:45 p.m.
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.
Associated Press Writer P. Solomon Banda in Centennial contributed to this report.
— Dan Elliott and Ivan Moreno, AP
Updated 2:50 p.m.
Authorities are in the process of investigating a male high school student who entered Arapahoe High School in Centennial, Colorado Friday and shot two people before allegedly fatally turning the gun on himself, Arapahoe County Sheriff Grayson Robinson said at a televised news conference at 3 p.m. MST Friday.
The name of the suspect, who was a current student at the high school, is not being released at this time and authorities are still investigating whether other suspects may have been involved.
Colorado Gov. John Hickenlooper said gunshots and "unspeakable harm" at schools had become "all too familiar" but officials' response Friday was "a world of change from how response used to happen."
"Those protocols have had to be used far too many times because of people who decided to even the score with a gun," said Robinson.
The suspect entered the school with a shotgun and immediately asked other students for the location of a specific teacher, said Robinson. When the teacher got word of the situation, he left the school.
One student suffered a minor gunshot wound, while another student is in serious condition from a gunshot wound and is undergoing surgery.
"We will conduct a thorough and complete investigation," said Robinson.
The investigation includes searching the suspect's home and areas where he may have had access to weapons, as well as interviewing his parents, friends and acquaintances.
Another press conference is scheduled for 6 p.m. MST/5 p.m. PST.
— KPCC's Jessica Hamlin, from televised press conference.
Updated 1:23 p.m. A male high school student suspected in a Friday shooting at Arapahoe High School in suburban Denver is dead, Arapahoe County Sheriff Grayson Robinson said at a televised news conference Friday.
The suspect died of "what appears to be a self-inflicted gunshot wound," said Robinson, who added that authorities are still investigating whether other suspects were involved.
The suspect mentioned he was targeting a specific teacher, who was transported away from the area.
Deputies responded to a call at the school at 12:33 p.m. Friday and recovered one weapon believed to be used by the suspect.
Littleton Adventist Hospital spokesman Jason Dunkel said a student was taken to a hospital in critical condition, according to the Associated Press.
Another student had a minor injury that may not be a gunshot wound, Robinson added.
Another press conference is scheduled for 3 p.m. MST/2 p.m. PST.
— KPCC's Jessica Hamlin, from televised press conference
Earlier: Police were responding to a report Friday of a shooting at a high school in suburban Denver, state emergency officials told the Associated Press.
Sheriff Grayson Robinson told Denver's KMGH TV that at least two were injured in the shooting. The sheriff said he also believed the shooter was still inside.
Colorado division of emergency management spokeswoman Micki Trost said her director went to Arapahoe High School in Centennial, the AP reported.
Police were outside the school, and students were seen walking toward the school's running track with their hands in the air. Television footage showed students being patted down.
The school is about 8 miles east of Columbine High School in Littleton, where two teenage shooters killed 12 classmates and a teacher before killing themselves in 1999.
Arapahoe also hosted a rally by 2012 Republican presidential candidate Mitt Romney in February 2012, according to Getty Images.
— AP with KPCC staff
This story has been updated.