• ABC News looks back at one of the worst mass shootings in U.S. history, nearly 5 years after twenty children and six adults were killed at Sandy Hook Elementary School in Newtown, Conn., on Dec. 14, 2012. <br> <br> Pictures of victims of the Sandy Hook Elementary School shooting are seen on artificial roses at a roadside memorial on Dec. 20, 2012, in Newtown, Conn.
    Brendan Smialowski/AFP via Getty Images
  • A police officer leads two women and a child from Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012. <br> <br> At around <a href="http://abcnews.go.com/GMA/video/newtown-connecticut-shooting-timeline-events-sandy-hook-elementary-17982876"target="external">9:30 a.m, a gunman, 20-year-old Adam Lanza, approached the school building</a> and unable to open the locked doors, shot his way through a window near the front office to gain entry to the school. He carried three weapons and multiple rounds of ammunition.
  • Two Connecticut State Police officers escort a class of students and two adults out of the school, Dec. 14, 2012. <br> <br> Lanza carried with him three guns, including a 9mm Sig Sauer and 10mm Glock. But most of his victims were shot multiple times using a Bushmaster .223 semiautomatic rifle.
    Shannon Hicks/Newtown Bee/Polaris
  • Police investigate at the scene of the shooting, Dec. 14, 2012.
  • A woman waits to hear about her sister, a teacher, following the shooting, Dec. 14, 2012.
    Jessica Hill/AP Photo
  • This undated crime scene photo shows the lobby area in Sandy Hook Elementary School. <br> <br> The gunman had shot and killed his mother, Nancy Lanza, 52, before going to the school.
    Danbury State Attorney
  • Law enforcement canvass the area around the school, Dec. 14, 2012. <br> <br> When he heard police coming toward him, Lanza shot and killed himself using a handgun in a classroom in the front hallway. <br> <br> Lanza's mother owned the guns that he used in the attack.
    Jessica Hill/AP Photo
  • President Barack Obama reacts as John Brennan briefs him on the details of the shooting, Dec. 14, 2012. <br> <br> The president later said during a TV interview that this was the worst day of his presidency.
    Pete Souza/White House
  • Connecticut State Police Lieutenant Paul Vance addresses a press conference on the situation following the shooting at the elementary school, Dec. 15, 2012.
    Emmanuel Dunand/AFP/Getty Images
  • Alissa Parker and her husband, Robbie Parker, leave the firehouse staging area after receiving word that their six-year-old daughter Emilie was one of the 20 children killed, Dec. 14, 2012.
  • Mourners gather for a candlelight vigil at Ram's Pasture to remember shooting victims, Dec. 15, 2012, in Newtown, Conn.
    Jason DeCrow/AP
  • A flag hanging in the business area of town waits for victim's names on Dec. 15, 2012
    Don Emmert/AFP via Getty Images
  • People arrive for the funeral services of six year-old Noah Pozner, who was killed in the shooting, Dec. 17, 2012, in Fairfield, Conn.
    Spencer Platt/Getty Images
  • The casket of James Mattioli, 6, leaves Saint Rose of Lima Church after his funeral, Dec. 18, 2012, in Newtown, Conn.
    Emmanuel Dunand/AFP/Getty Images
  • A child's message rests with a memorial for shooting victims, Dec. 16, 2012.
    Jason DeCrow/AP
  • Guy Bacon, center, walks with family members out of Christ the King Lutheran Church following funeral services for his sister, Charlotte Helen Bacon, one of the students killed, Dec. 19, 2012, in Newtown, Conn.
    Julio Cortez/AP Photo
  • Friends and co-workers hold a sign as the hearse and funeral procession for James Mattioli, 6, who died in the shooting, approaches the St. John's Cemetery, Dec. 18, 2012, in Darien, Conn.
    Craig Ruttle/AP
  • People gather outside the White House to participate in a candle light vigil to remember the victims of the elementary school shooting on Dec. 14, 2012, in Washington, D.C.
    Alex Wong/Getty Images
  • Gun control supporters take part in a candlelight vigil at Lafayette Square across from the White House, Dec. 15, 2012. <br> <br> The families of nine victims who were killed and one teacher who survived are part of a lawsuit which argues that Remington, the parent company of the gun manufacturer, Bushmaster, knowingly marketed a military weapon to civilians. <br> <br> They <a href="http://abcnews.go.com/US/sandy-hook-dad-utmost-faith-case-families-hope/story?id=51137338"target="external">are urging Connecticut's highest court to reinstate the suit</a> which was dismissed in 2016.
    Mandel Ngan/AFP via Getty Images
  • President Barack Obama speaks in the East Room of the White House on Jan. 5, 2016, about steps his administration is taking to reduce gun violence.
    Pablo Martinez Monsivais/AP Photo
  • A mourner attends a candlelight vigil at Ram's Pasture to remember shooting victims, Dec. 15, 2012, in Newtown, Conn.
    Jason DeCrow/AP