"He knew he was the target and he left that school in an effort to try to encourage the shooter to also leave the school," the sheriff said. "That was a very wise tactical decision."
The shooter then shot two students, including a girl who was in serious condition. The other student suffered minor injuries and was expected to be released from the hospital Friday night, he said.
A suspected Molotov cocktail was also found inside Arapahoe High School, the sheriff said. The bomb squad was investigating the device.
Deputies and police officers who responded to the school immediately entered to engage the gunman as students were kept locked in their classrooms.
Several other school districts in the Denver area went into lockdown.
The practice of sending law enforcement directly into an active shooting was developed in response to another Colorado school shooting that shook the nation: In 1999, two teenage shooters killed 12 classmates and a teacher at Columbine High School before killing themselves.
Arapahoe High School is just 8 miles (13 kilometers) from Columbine.
More recently, a shooter killed 12 people at a movie theater in Aurora, another Denver suburb, in the summer of 2012.
Earlier this year, Colorado's Democratic Legislature and governor approved a slate of gun control measures, including ammunition magazine limits and expanded background checks.
Two state lawmakers were ousted in recall elections after voting for the measures, and a third resigned to avoid a recall. The backlash was a reflection of the divide in a state known for its hunting and where gun ownership is a treasured right.
At 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 but now believe the gunman acted alone.
For one mother who rushed to Arapahoe High, it was her second experience with a school shooting.
Tracy Monroe, who was reading text messages from her 15-year-old daughter inside Arapahoe, said she had step-siblings who attended Columbine. The teacher killed in the Columbine shooting, Dave Sanders, was a family friend.
"We didn't think it could happen in Colorado then, either," Monroe said.
Monroe said she got the first text from her daughter, 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 that she was safe.
Students were seen walking toward the school's running track with their hands in the air. Television footage showed police patting students down.
Colton Powers, 14, said he heard three shots in a classroom. The shots sounded far away, he said.
"We all ran to the corner of the room and turned off the lights and locked the door and just waited, hoped for the best," he said. "A lot of people, I couldn't see, but they were crying.
His mother, Shelly Powers, said she first got news 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. I was crying all the way here," she said.
