(CNN)In a matter of minutes, thousands of students at Marjory Stoneman Douglas High School went from thinking they were part of a fire drill to hiding inside closets and bathrooms to escape deadly gunfire.
What happened, moment by moment, in the Florida school massacre
Authorities say Nikolas Cruz opened fire inside the Florida school on Valentine's Day, killing 14 students and three teachers.
This is how events unfolded in one of the 10 deadliest mass shootings in modern US history, according to law enforcement authorities, school officials and witnesses.
On Thursday, the Broward County Sheriff's Office released more information about what happened that day, including details and quotes compiled from school video and radio traffic.
All times are Eastern.
• Cruz takes an Uber car.
• Uber car drops Cruz near Marjory Stoneman Douglas High School in Parkland.
• A witness allegedly spots Cruz arriving in a goldish-colored vehicle and alerts a colleague via radio that Cruz was "walking purposefully toward the 1200 Building." Within a minute, he hears gunshots and reports a code red situation -- an active shooter.
• The suspect enters the school's east stairwell of Building 12, with a rifle inside a black soft case. When he exits the stairwell, he pulls the rifle out of the case and begins firing.
• He allegedly sets off the school's fire alarm a minute later, prompting the evacuation of some 3,000 students and staff. Investigators believe the suspect wanted to draw people out of classrooms to increase the number of casualties.
• The suspect begins shooting into four classrooms. He takes another stairwell to the second floor and allegedly shoots one victim.
• The shooter uses a .223-caliber AR-15-style rifle to carry out the massacre.
• As the school goes on lockdown, students take cover inside classrooms. Others jump over fences and escape to take shelter in a nearby Walmart. Some students text their loved ones and others post on social media images of chairs overturned in classrooms and floors stained with blood.
• Twelve people are killed inside the building, two outside and one in the street. Two others die at the hospital.
• School resource deputy Scot Peterson is near the school's administration building.
• First 911 call is made, to city of Coral Springs communications center.
• Peterson is seen on video heading to Building 12. He advises: "Be advised we have possible, could be firecrackers, I think we have shots fired, possible shots fired, 1200 Building."
• Peterson is near the southeast corner of Building 12 and northeast corner of Building 7. He appears to stay here during the shooting.
• Coral Springs Fire Department is dispatched to school.
• A Broward sheriff's deputy says shots are heard by the football field. Peterson calls for the school to be locked down.
• The deputy reports students by the football field thought there were firecrackers, "but we're not sure." Peterson tells the deputy, "We also heard it's by, inside the 1200 Building."
Coral Springs police officer asks dispatch about possible active shooter. Officer is told 911 lines are "blowing up."
• First Coral Springs officer arrives at school.
• Peterson says to make sure no one comes inside the school.
• The gunman takes the building's east stairwell to the third floor, where he drops the rifle and backpack. Then, he runs down the stairs.
• The suspect exits Building 12, runs toward the school tennis courts.
• Seconds after Cruz leaves the building, Peterson says, "Broward, do not approach the 12 or 1300 building. Stay at least 500 feet away at this point."
• Cruz continues running and eventually mixes in with the students and staff running out of the school.
• On the Coral Springs radio, which cannot be heard by Broward deputies, the suspect is described as a white male with an ROTC uniform burgundy shirt.
• Two Broward County deputies try to enter the 13 Building, but are unable because it is locked.
• A description of the gunman is sent out by Broward County as a male in a hoodie with an AK-47 or AR-15.
• Coral Springs officers are told three people are down in room 1216.
• Broward deputy says he is told of a suspect possibly inside a three-story building and wearing an ROTC uniform.
• Four officers, possibly from Coral Springs, and two Broward deputies enter Building 12 through a west-facing door, about 11 minutes after the first shots.
More law enforcement officers enter Building 12 through a door on the east side.
• The gunman arrives at a nearby Walmart store, buys a drink at a Subway there and walks out.
• Suspect goes to a nearby McDonald's, where he sits down for less than a minute before he walks out.
• The FBI and dozens of police vehicles respond to the shooting.
• Video shows some students lying on the ground, being treated for injuries and moved into ambulances.
• White House press secretary Sarah Sanders says President Donald Trump was briefed on the incident and is monitoring the situation.
• The gunman is still at large.
• The school begins dismissing students. Buses take them to a hotel, where they reunite with their families.
• A Coconut Creek Police officer searching for the suspect in a nearby neighborhood notices that someone walking on the side of the road is wearing clothing matching the suspect's description. Details about the suspect had been transmitted via law enforcement radio.
• A witness who had seen Cruz arriving at the school prior to the shooting identifies him at the arrest location.
• Authorities had also identified the suspect using school security videos.
• The officer approaches the suspect and arrests him without incident.
• He is taken to the Broward Health North hospital because he has "labored breathing." Cruz is treated and released to police custody.
• President Trump tweets, "My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school."
• Law enforcement searches the school's buildings to evacuate students from classrooms.
• Broward County Public Schools Superintendent Robert Runcie says the school received no warnings and that he was not aware of any threats against the school.
• Gov. Rick Scott travels to Broward County to be briefed about the shooting.
• Authorities identify the alleged gunman as Cruz -- a 19-year-old former student at the high school. He arrives at the Broward County Sheriff's Office wearing a hospital gown and is escorted inside by deputies.
6:30 p.m. to end of day
• Cruz tells police he was the gunman and says he discarded the firearm.
• School district officials say the school will remain closed for the rest of the week.
• The FBI sets up a website for students and others who were inside the school to upload videos and photos of the shooting.
• Law enforcement officials search Cruz's home and another undisclosed location.
• Cruz, who is being held without bond, makes his first court appearance in Broward County via jail video. He said nothing except to confirm his name.
• Cruz is charged with 17 counts of premeditated murder in connection with the shooting.