Multiple major school districts across the United States are reporting that they received threats similar to the ones that were made against schools Los Angeles and New York earlier this week.

Schools in Miami, Houston and Dallas all reported receiving the threats on Wednesday evening. The threats weren’t determined to be credible and schools in those cities stayed open Thursday.

School officials in Los Angeles canceled all classes on Tuesday after receiving a threat that involved backpacks and other packages. The threat was ultimately determined to be a hoax.

New York Police Commissioner Bill Bratton told reporters at a Tuesday news conference that their schools got a similar threat, but determined it wasn’t serious. Classes went on as planned.

Speaking at a news conference Thursday, Miami-Dade County Public School Superintendent Alberto M. Carvalho said that someone emailed board members in multiple school districts on Wednesday night with the exact same message. After speaking with various law enforcement agencies, the threat wasn’t deemed credible and Thursday went on as “a regular school day.”

Still, the district increased its law enforcement presence in schools.

Carvalho said at the news conference that schools in Broward County, Florida, and Long Beach, California, received similar threats. The Houston Independent School District and Dallas Independent School District also got similar threats, officials there said in statements. The Orange County (Florida) Public Schools were also threatened, according to their Facebook page.

“At this time, we do not believe the threat is credible, but as a precautionary measure law enforcement officers are in the process of conducting random sweeps of school district buildings to ensure student safety,” the Houston Independent School District said in a statement.

The Dallas Independent School District said bomb-sniffing dogs were used in their sweeps.

The threats are being made against some of the largest school districts in the country.

According to American School & University Magazine, New York and Los Angeles are America’s largest and second-largest school districts in terms of enrollment, respectively. Miami-Dade ranked fourth, Broward County was sixth, Houston was seventh, Orange County was 10th and Dallas was 14th. Together, those seven districts educate close to 3 million students every day.