PARKLAND, Fla. (CBSNewYork/CBS News) — A hurricane-proof window may have prevented the shooter in a deadly Valentine’s Day massacre at a high school in South Florida from killing additional people, sources tell CBS Miami’s Jim DeFede.

Investigators believe 19-year-old Nikolas Cruz tried to make a sniper’s nest by shooting out a window in a third-floor stairwell.

He fired 16 rounds into the glass, but the hurricane-proof material didn’t shatter. Sources tell CBS Miami Cruz then reloaded only to have his gun jam. With police closing in, Cruz dropped the weapon and exited the school with other students.

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Additionally, a federal law enforcement source with direct knowledge of the investigation told CBS News on Tuesday the suspected gunman had swastikas on ammunition magazines he brought into Marjory Stoneman Douglas High School on Feb. 14.

Cruz had 180 rounds of ammunition left, a source confirmed to CBS News. Police said Cruz told them he had “brought additional loaded magazines to the school campus and kept them hidden in a backpack until he got on campus to begin his assault.”

Cruz has been charged with 17 counts of premeditated murder. On Wednesday, the same teens who raced away from their school in a panic two weeks ago are slated to return to the scene of the terror and heartbreak.

“Going back to our normal lives will definitely be hard for me,” Stoneman Douglas student Carlos Rodriguez said.

As they arrive on campus, the view of the crime scene will be concealed by signs of support and messages of love.

“It’s not gonna be the same hallways,” Rodriguez said. “It’s not gonna be the same smiles. It’s definitely gonna be rough just to be in school and know that’s where 17 people died.”

There will be extra security and counselors waiting, along with an emotional principal who promised to hug each and every one of the students.

In their time of grief, the survivors have become activists. Some of the teens were on Capitol Hill again Tuesday, meeting with lawmakers and making their case for tougher gun control laws.

Republican leaders say they’ll be focusing on fixing the background check system rather than big reform laws. Democrats say the GOP must do something, even if it goes against the National Rifle Association.

As much as they’re talking about change, it appears so far Congress can’t seem to find common ground.