The Texas Education Agency confirmed that a Splendora High School teacher, who was charged in an alleged hoax on campus, is now under investigation.
Police found no threat to Armstrong Elementary after responding to a reported fight nearby, that sent the school into lockdown.
The campus and three others nearby were placed under "secure status" on Tuesday afternoon. One person was detained.
Authorities said more than 2,700 students were at school when a threat was reported, followed by confusion. One student told ABC13 she hid in a classroom closet with classmates.
District officials said there would be extra officers at the campus after someone made a bomb threat demanding ransom money Thursday morning.
HISD said the man was seen behaving strangely, and teachers immediately reported it. The district has not responded to questions about whether the man actually went inside the school building.