It's been 20 years since two teenage boys dressed in black trench coats shot and killed 13 people and injured more than 20 others at Columbine High School.

The events at the Littleton, Colorado high school on April 20, 1999 shocked the nation. Hearing the word "Columbine" still summons mental images of that day — students, led by police, leaving their school with hands above their heads and people embracing each other in grief.

In the years that followed, other names joined the list of schools that suffered similar tragedies. Virginia Tech. Sandy Hook. Parkland. Santa Fe. Columbine was the worst at the time but not the first: The list includes more than 1,300 shootings in K-12 schools since 1970 in the United States, according to the Naval Postgraduate School's Center for Homeland Defense and Security.

In Wisconsin, five school shootings have occurred in the 20 years since the massacre at Columbine. In some cases, ordinary people became heroes as they stepped up to protect others in danger. In other cases, questions still remain about the shooters' motives.

This is a closer look at those five Wisconsin tragedies.

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Feb. 20, 2002: Vincent High School, Milwaukee

Sixteen-year-old Joseph Johnson, a Washington High School student, was shot and killed in the parking lot of Vincent High School during a fight that broke out after a basketball game between the rival schools, according to an Associated Press report.

Phillip D. Jackson Jr., then 22, was charged with the teen's death. Authorities said he fled on foot after opening fire in the parking lot

Jackson was caught two months later after shooting a man who witnessed him fire his gun into the crowd, according to the report. Jackson was a passenger in the witness' van the night of the game.

Jackson is serving a 54-year prison sentence at the New Lisbon Correctional Institution.

Sept. 29, 2006: Weston High School, Cazenovia

As Weston High School was preparing for homecoming weekend, Eric Hainstock, then 15, was planning an attack.

Hainstock entered the school with a shotgun early that Friday. He told police that other students had been bullying him and that Principal John Klang and other teachers wouldn't do anything about it, according to The Associated Press.

Hainstock had also received a disciplinary note from Klang the day before the shooting after Hainstock was caught with tobacco in school. The note said Hainstock would likely receive an in-school suspension, according to the AP.

After Hainstock entered the school, he yelled that he had a gun. A custodian at the school immediately grabbed the shotgun and Klang tackled the student. Hainstock pulled a handgun from his pants during the struggle and shot Klang in the head, chest and leg. Witnesses said Klang was able to disarm Hainstock despite being injured. Klang later died at a Madison hospital.

Hainstock was charged with first-degree intentional homicide and is serving a life sentence at the Wisconsin Secure Program Facility in Boscobel.

Students honored Klang after the shooting by wearing shirts calling him a superhero. A teacher at the school at the time indicated that it was no surprise that Klang did everything he could to protect his students, saying he was always thinking of them first.

Nov. 29, 2010: Marinette High School

The day classes resumed at Marinette High School after Thanksgiving break, Samuel Hengel, 15, brought two handguns, a knife and a duffel bag full of ammunition into a social studies class and took 25 students and a teacher hostage.

A student in the classroom said they had been watching a movie when Hengel shot at the projector and shot another round near their teacher, Valerie Burd. The rest of the school went on with the day, not knowing what was happening in the basement classroom.

Former Principal Corry Lambie went to the classroom around 3:40 p.m. after hearing from a concerned father who hadn't heard from his daughter, according to reports from the Green Bay Press Gazette. As Lambie tried to enter, Hengel pointed a gun at him and calmly told him to leave, according to the report.

Hengel didn't make any demands in the more than six hours he held the students against their will. He would communicate with law enforcement only through Burd, whom he allowed to talk to police over the phone.

Authorities called Burd a hero for remaining calm during the hostage situation. Things could have ended differently if not for her keeping a "cool head," they said.

Hengel fired a few shots around 4 p.m. that day and again around 8 p.m. Shortly after, Hengel shot himself as police stormed the room, according to the report. No hostages were injured.

Hengel died the next morning at St. Vincent’s Hospital in Green Bay. He was the only fatality. His motive for taking more than two dozen people hostage is still unknown.

MILWAUKEE JOURNAL SENTINEL ARCHIVE: Teen gunman in school hostage drama dies

Jan. 15, 2015: Wisconsin Lutheran High School in Milwaukee

A fight that broke out in the parking lot of Wisconsin Lutheran High School ended as Michael Riley, then 37, shot and injured two people, according to a 2015 report from the Milwaukee Journal Sentinel.

The shooting was the result of an earlier fight between two girls attending a basketball game, according to the report. One girl was from Wisconsin Lutheran High School and the other girl was from another school.

The girls met up in the parking lot after being escorted out of the game separately. Their families were parked next to each other and the girls' fathers got involved when the fight became physical. Witnesses reported seeing Riley pull out a gun and fire a shot that went through the other father's knee and hit a teacher in the foot, according to the report. There were no fatalities.

Riley was later charged with first-degree reckless injury, first-degree recklessly endangering safety, being a felon in possession of a firearm and discharging a firearm in a school zone, according to the report. He is serving a six-year sentence at Redgranite Correctional Institution.

MILWAUKEE JOURNAL SENTINEL ARCHIVE: Milwaukee man charged in Wisconsin Lutheran High School shooting

April 23, 2016: Antigo High School

A night meant for students to dance and have fun with friends turned deadly when 18-year-old Jakob Wagner shot at a group of students as they left Antigo High School's prom late that Saturday night.

Antigo police officers Andrew Hopfensperger Jr. and Ryan Bula, along with a police dog, were patrolling school grounds that night. Hopfensperger, who was near the students during the incident, shot back at Wagner, severely injuring him.

Wagner died at Aspirus Wausau Hospital early the next morning. He was the only fatality of the shooting. One other student was seriously injured and another suffered less serious wounds.

An autopsy of Wagner found he had his arm extended when he was shot, which is consistent with Hopfensperger's statement that Wagner had his gun aimed at the officer.

Some of Wagner's classmates described him as friendly, kind and gentle, while others described him as troubled. Some said he had been bullied since middle school and had an unusual interest in guns.

Jennifer Fisher spoke with USA TODAY NETWORK-Wisconsin in 2016. Though she wasn't close with Wagner, she said they had known each other for about a decade. His attempts to make friends and join conversations were largely ignored by his peers, she said.

"He wasn't a monster. He just wanted to be friends," Fisher said in 2016. "Something must have happened because he wasn't like that."

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The Milwaukee Journal Sentinel and The Associated Press contributed to this report.