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Denver authorities on Saturday searched the home and car of the gunman who opened fire at a high school, critically injuring a 17-year-old girl before he shot and killed himself.

Arapahoe County Sheriff Grayson Robinson toldThe Denver Post investigators were interviewing students, teachers and staff members Saturday in an effort to learn every tiny detail about the shooting at Arapahoe High School. Officials have identified the gunman as 18-year-old Karl Pierson.

"We will interview hundreds of people," Robinson said Saturday morning. "We are going to do it right."

Tracy Murphy, a librarian and debate team coach, is believed to have been the gunman's target in the shooting.

According to KUSA-TV in Denver, Murphy implemented "active-shooter protocols" after he learned that Pierson was armed with a shotgun and asking for him on Friday. Murphy then left the scene, a move Robinson said may have helped to limit the potential carnage.

Robinson also heaped praise Saturday on a school resource officer who responded immediately after the first shots were fired.

"He found (Pierson's) body within five minutes," Robinson told the Post. "Not only did he know what he was doing but he was very courageous."

On Friday night, Robinson would not elaborate on any possible motive except to say Pierson, who was found dead of a self-inflicted gunshot wound, had had a "confrontation or disagreement" with the teacher.

An Arapahoe County deputy escorted Murphy home to pick up a few belongings on Friday night. Murphy declined to talk about the shooting. What he has to say about his interactions with Pierson likely will be a key part of the investigation.

The 17-year-old girl who suffered a gunshot wound and remained in critical condition on Saturday. Two other students suffered minor injuries.

On Saturday, students were able to return to the high school for the first time since being evacuated during the shooting Friday afternoon. They picked up their cars, but they were not allowed to go inside the school, which remains a crime scene. The school was surrounded in yellow crime scene tape.

It is unknown when students will be allowed back into the school to pick up their backpacks, cellphones or other items left behind. Classes are canceled on Monday. Finals were scheduled to start Tuesday, but those also have been canceled.

"Communication about final exams will come as we have more information, but they will not occur in any format until after the holiday break," according to a letter from the Arapahoe High School Administrative Team.

The shooting — on the eve of the first anniversary of the Newtown school massacre, in which 20 students and six staffers were killed — sent scores of terrified students and staffers at Centennial's Arapahoe High School scurrying at about 12:30 p.m. Police and other first responders quickly mobilized to surround the 2,220-student school.

The gunman also brought two Molotov cocktails inside the school and exploded one, KUSA-TV reported. The other was found and removed by the bomb squad.

The incident unfolded when the armed student entered the west side of the school from a student parking lot. He told other students he was interested in confronting a specific teacher.

Many students locked themselves in classrooms until first responders arrived. Some said they heard several gunshots in a hallway near the school library.

"We were shaking, we were crying, we were freaking out,'' 9th grader Whitney Riley told CNN.

Jessica Girard was in math class when she said 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.

Contributing: KUSA-TV, Denver; Associated Press