Medical personnel tend to a victim following a shooting at Marjory Stoneman Douglas High School in Parkland, Fla., Feb. 14, 2018.
John McCall, South Florida Sun-Sentinel via AP

The deadliest high school shooting in modern American history occurred about 2 p.m. ET on Wednesday at Marjory Stoneman Douglas High School in Parkland, Fla., about 30 miles northwest of Fort Lauderdale.

Here's what we know:

How many were killed and injured?

At least 17 people were killed, 12 of them inside the school. Three more were killed just outside the school and another two died of their injuries after being taken to the hospital, Broward County Sheriff Scott Israel said.

The victims included both students and adults. 

A total of 17 people, including the suspected gunman and the two victims who died later, were taken to local hospitals. At least three of them are in critical condition and three are stable. The suspect was treated and released into police custody, doctors said. 

"It's a horrific situation. It's just a horrible day for us," said Broward Superintendent Robert Runcie. "...This is a day we prayed would never happen in our county." 

'There are really no words': 17 dead in Florida high school shooting; former student in custody

Who is the gunman?

Authorities identified the gunman as Nikolas Cruz, 19. Cruz, a former student at Marjory Stoneman Douglas High School, had been expelled from the school and did not graduate, authorities said. 

At least one teacher said he had been flagged as a potential threat and that he was a big enough concern that administrators had banned Cruz from campus. 

Jim Gard, a math teacher at Marjory Stoneman Douglas High School, said Cruz had been aggressive towards students in the past. 

A former classmate, Jillian Davis, said Cruz had a hot temper and a history of making dark, gun-related jokes. 

“Finding out it was him makes a lot of sense now,” Davis said.

Cruz's first name also appears as Nicolas in some official records.

More: Suspect in fatal Florida school attack is former student with 'anger' issues

What happened:

Broward County Public Schools reported the shooting began near dismissal time, when staff and students said they heard the sounds of gunfire. 

Some students said a fire alarm sounded before the gunfire. Police say Cruz began the rampage outside before opening fire inside the high school. 

Sen. Bill Nelson, D-Fla., told MSNBC that Cruz wore a gas mask and tossed smoke grenades to draw students out into the hallways. 

The school was placed on lockdown. Television footage showed students running from the school with their arms in the air, tossing backpacks into a pile. Some students were being treated on the sidewalks and loaded into ambulances.

Cruz was at large before being taken into custody off the school's campus roughly two hours after the shooting.

Police said he was armed with an AR-15-style, semiautomatic rifle and "countless" magazines.

More: Florida school shooting is the 6th to injure students this year

Read more: 20 years in, shootings have changed schools in unexpected ways