THE teenage gunman who killed a student and a teacher at a St. Louis high school left a note in his car saying his feelings of loneliness were "a perfect storm for a mass shooting," the city's police commissioner said on Tuesday.

Police found a blue notebook in the car that the killer, Orlando Deshawn Harris, drove on Monday to the Central Visual and Performing Arts High School, where he fatally shot the 61-year-old teacher and 16-year-old female student, while wounding or causing injuries to seven other people.

Harris was killed in an exchange of gunfire with police.

Describing the handwritten note as a "manifesto," Police Commissioner Michael Sack read a portion of the document to the media at a press conference on Tuesday.

"I don't have any friends," Sack quoted the note as saying "I don't have any family. I never had a girlfriend. I never had a social life. I've been an isolated loner my entire life. This was the perfect storm for a mass shooter."

Sack said Harris may have suffered from mental illness and the note shed some light on his state of mind.

"He feels isolated, he feels alone," Sack said. "Quite possibly angry and resentful of others who have, it appeared to him, to have healthy relationships, so a desire to lash out."

Harris, 19, had no previous criminal history, Sack told reporters. He was armed with an AR-15-style rifle and he had 600 rounds of ammunition.

The shooting is one of many school shootings that have left dozens dead and wounded across the United States this year alone. One of the deadliest took place in May when a gunman killed 19 children and two adults in Uvalde, Texas.

SOUNDS OF GUNSHOTS

St. Louis authorities did not disclose how Harris, a former student at Central, entered the locked building at about 9:10 a.m. on Monday. The rampage lasted about 15 minutes before police stormed the school.

In a statement on Tuesday, police said officers found Harris barricaded inside a third-floor classroom. Harris did not comply with officers' commands to drop his weapon and surrender. He fired his rifle, and officers shot and killed him.

Many of the students had barricaded themselves in classrooms and some jumped out windows, media reports said.

In addition to the two fatalities, four teenagers suffered gunshot wounds and three other youths sustained other injuries.