Florida school shooting suspect fired at hurricane-proof window in attempt to set up perch: Official

— -- Florida school shooter Nikolas Cruz allegedly fired at a third-floor window of Marjory Stoneman Douglas High School in an apparent attempt to set up a perch from where he could shoot students who were outside, an official briefed on the investigation told ABC News today.

Cruz's gunshots couldn't break through the window because it's made of hurricane-proof glass, the official said. Investigators believe the 19-year-old intended to position himself above ground so he could open fire at those fleeing the school in Parkland, Florida.

Robert Runcie, the Broward County Public Schools superintendent, said in an interview today with ABC News' Victor Oquendo that bullets were fired "through the window" on the third-floor of the freshman building. Although the investigation is ongoing, Runcie said it appears Cruz allegedly tried to purposely blow out that window.

"I had a full briefing from the crime investigators and law enforcement, so that is one of the assumptions and apparently looked like that that was attempted," Runcie said.

Seventeen people were killed and dozens more were injured in the Feb. 14 shooting at Marjory Stoneman Douglas High School. The AR-15 semi-automatic rifle allegedly used in the massacre was legally purchased by Cruz a year ago, authorities said.

A law enforcement source told ABC News that Cruz left behind nearly 180 rounds of ammunition -- or about six fully-loaded 30-round magazines -- when he allegedly abandoned his rifle and fled the school. The source said at least one of the magazines recovered at the scene had a swastika on it.

Classes at Marjory Stoneman Douglas High School resumed today.

Cruz was apprehended and charged with 17 counts of premeditated murder. He is being held without bond at the Broward County Jail.

After Cruz was apprehended and read his Miranda rights, he allegedly stated to authorities that he was the gunman who entered the school with an AR-15-style rifle and began shooting students he saw in the hallways and on school grounds, according to a probable cause affidavit from the Broward County Sheriff's Office. He also allegedly stated that he brought additional loaded magazines to campus and kept them hidden in a backpack until he got there and began the attack, the affidavit states.

Cruz had been expelled from Marjory Stoneman Douglas High School for unspecified disciplinary reasons, authorities said. He attended the school from Jan. 13, 2016, to Feb. 8, 2017, according to school records obtained by ABC Miami affiliate WPLG.

ABC News' Jack Date contributed to this report.