By Nicole Chavez, CNN

(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 gunfire.

Authorities say Nikolas Cruz allegedly opened fire inside the Florida school on Wednesday, killing at least 17 people, both adults and children.

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. All times are Eastern.

Wednesday February 14

About 2:30 p.m.

• Cruz arrives at the Marjory Stoneman Douglas High School in Parkland, Florida. He is wearing a gas mask and carrying smoke grenades. It's unclear if the smoke grenades were used.

• He allegedly sets off the school's fire alarm, 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 shooter uses a .223 caliber AR-15 firearm to carry out the massacre.

• As the school goes on lockdown, students take cover inside classrooms. Others jump over the school's 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.

2:53 p.m.

• The Broward County Sheriff's Office begins investigating reports of an active shooter at the school. When deputies arrive, they see hundreds of students fleeing.

• The alleged gunman mixes in with the students and staff running out of the school.

3:11 p.m.

• 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.

3:15 p.m.

• The gunman is still at large.

3:40 p.m.

• The school begins dismissing students. Buses take them to a nearby hotel where they reunite with their families.

3:50 p.m.

• 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."

3:53 p.m.

• Law enforcement searches the school's buildings to evacuate students from classrooms.

4:10 p.m.

• Authorities find Cruz in a nearby neighborhood and arrest him without incident. They identify him using school security videos.

• He is taken to the Broward Health North hospital because he has "labored breathing." Cruz is treated and released to police custody.

4:20 p.m.

• 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.

5:18 p.m.

• Florida Gov. Rick Scott travels to Broward County to be briefed about the shooting.

6 p.m.

• Authorities identify the alleged gunman as Nikolas 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

• 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.

Thursday, February 15

• Cruz is expected to make his first court appearance in Broward County.