Similar false calls were reported to more than a dozen other schools in Texas and California, officials said.
READ MORE: 13 Investigates sources confirm calls for multiple Texas school threats came from same phone number
Multiple Texas school threats came from same phone number, sources say
Sources told ABC13 that the same phone number called dispatchers in Texas and reported false claims of active school shooters and alleging there to be victims.
In addition to Houston and Waco, ABC13 sources said false reports were also made in Pflugerville and a fourth Texas school district.
Initially, the caller told police there were 10 victims at the high school, according to investigators.
The Houston Police Department is the lead agency for the false call to the Greater Heights-area school.
Parents and students told ABC13 they found themselves struggling to find answers during the frantic moments that followed the fake shooter call.
"It was chaotic and it was really bad because there was a bunch of things happening," student Denice Cruz said. "We didn't know what was happening."
The 11th-grader said she first thought the action at Heights High School was a drill, but once police showed up telling her and fellow students to hide behind desks, she knew something serious was happening.
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While the call about an active shooter turned out to be fake, Cruz said not knowing what was happening added to the anxiety everyone felt at the school.
"They didn't give us any updates. All we knew what was happening was online through Instagram and Google and everything that was going around," Cruz said.
Some parents said what they believe was a slow pace of information left them very worried for their kids' safety.
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During the chaos that ensued after the false call came in, even cellphones were useless, parents told ABC13.
"(Tuesday), I went through hell not being able to communicate with my child. She has a phone, but I could not communicate with her," parent Jaimie Tello said. "It was just -- I am just happy it was a hoax, and it wasn't real."
The FBI said it's working around the clock to find out who made that call and hold them accountable.
"We will find you. We will locate you. We will hold you accountable," Acting Special Agent-In-Charge Christopher Raya said.
Several resources were used to respond to the call.
"When these things typically go out, it's all hands on deck from all agencies until we can confirm whether the threat is real, as we saw (Tuesday). Thankfully it was a hoax," Raya said.
On Tuesday, a Houston ISD spokesperson said the district's alert to parents went out 23 minutes after the district became aware of the situation.
HISD's Chief of Engagement Max Moll sent ABC13 a statement saying, "HISD was able to disseminate communications containing verified information to Heights HS families within 25 minutes of becoming aware of the situation thanks to the efforts of all involved."
HPD Chief Troy Finner said his focus on Tuesday was safety.
"We love our parents, but it's a process," Finner said. "You have to make things safe. Because if you don't and somebody's actually shooting somebody in that school, then that is the worst case scenario."
Now, the focus is on finding who did this.
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HISD incident comes amid increase in violent threats for district