HOUSTON – More schools in the Houston area beefed up security Monday after another rash of threats on social media from accounts featuring creepy-looking clowns.

The threats, which have been circulating nationwide on websites like Facebook, Twitter and Instagram, prompted a response from some local school districts on Friday. Those responses increased Monday.

In many of the messages sent to parents, school officials said investigators have determined that the threats are “not credible,” but extra measures were being taken to ensure the safety of students and staff as a precaution.

On Monday, officials in Spring Independent School District, Alvin ISD and Goose Creek Consolidated ISD said they called in extra security.

Schools that have received threats:

Katy ISD

Klein ISD

Alvin ISD

Sheldon ISD

Spring ISD

Bay City ISD

Humble ISD

Houston ISD

Cy-Fair ISD

A phone message sent to parents of Houston ISD students did not mention increased security, but said the safety of students is a top priority.

AUDIO: HISD message to parents about clown threats

“Over the last several days, non-specific threats to schools in Houston and surrounding school districts have been posted to social media. Please know all students are safe. HISD police and other law enforcement agencies have determined the threats were not credible. Out of an abundance of caution, police are monitoring the situation. Teaching and learning are continuing as usual. Though the threat is not believed to be credible, we are taking every precaution possible to ensure safety. Please know we take all threats seriously as the safety or our students and staff is always or top priority.”

A 14-year-old student was arrested Monday in connection to a threat at one HISD school, according to the district.

In a Facebook post, Klein ISD officials called the threats a “social media hoax,” but said police and administrators will remain diligent throughout the week.

Klein ISD is aware of posts made on social media sites regarding perceived threats toward several state and local... Posted by Klein ISD on Monday, October 3, 2016

Katy ISD issued a statement Monday that read in part, "The District is aware of non-specific social media threats involving clowns that have been made towards some school districts in the Houston area. We assure you that Katy ISD Police is closely monitoring this matter and has found no evidence of any credible threats against our schools at this time. However, as an added safety measure, KISD PD will provide additional patrols across all campuses this week."

Sheldon ISD officials said in a Facebook post that “C.E. King” was mentioned in a threat posted on Instagram. They said additional security measures will be in place at both C.E. King Middle School and C.E. King High School. There were significant absences from both schools and some parents picked up their children out of fear from the threats.

Sheldon ISD administrators were informed on Saturday of another potential Clown Instagram Hoax - however, this time... Posted by Sheldon ISD on Sunday, October 2, 2016

On Friday, Westside High School sent a letter to parents about a threat posted on a Facebook page used by a group called “Aint Clownin Around.” That page has since been deactivated.

"This is Principal Stewart at Westside High School to inform you of an anonymous threat social media made against schools, including ours, which has been circulating in the region and in other states," the letter said. "We immediately notified (Houston Independent School District) police, and officers determined the threat was not credible. However, HISD police have assigned extra officers to patrol our campus as a precautionary measure."

The letter said visitor access was restricted Friday to only essential school staff, and extra HISD police officers were patrolling the campus.

A similar letter was also sent to parents of students at Northside High School, Tanglewood Middle School and the Spring Independent School District.

IMPORTANT MESSAGE FROM SPRING ISD --- Social Media Hoax Dear Spring ISD Parents and Guardians: We have been made... Posted by Spring ISD on Sunday, October 2, 2016

In a letter sent Friday to parents in the Bay City Independent School District, officials said a precautionary lockdown was enacted at Bay City High School and Cherry Elementary School because of the rumors.

Fort Bend ISD police sent a letter to parents reassuring students' safety and reminding parents that children should be mindful of what they are posting:

"I am emailing you to make you aware of social media posts that have been circulating nationwide and in the Houston-area over the last few days as part of a social media hoax, fueling concerns about the safety of our students. The safety of our students is the District’s number one priority, and I want to assure you that the Fort Bend ISD Police Department is in constant contact with our school administrators to ensure that our schools are safe learning environments.

"Although the schools referenced in the Houston area and nationwide are at the middle and high school levels, I am sharing this message with all of our parents so they are aware, in case they come across these posts on social media.

"The posts that have been circulating come from accounts that depict photos of clowns, though the names of the accounts vary. Fort Bend ISD Police and other law enforcement agencies are working with the social media platforms to have the accounts deactivated and the account information preserved to assist in investigations.

"The Fort Bend ISD Police Department is investigating each account that is brought to our attention, and there has been no evidence that any of these threats are credible. Our investigators are following up on leads to determine the source of the social media posts, as making a threat toward a school is a serious criminal offense.

"School is continuing as normal at our campuses, and as always, officers are present at every middle and high school. We are reminding visitors that they must check-in at the front office at all times.

"We encourage all students to report anything or anyone that makes them feel uncomfortable, including social media posts that could be considered as a threat against student safety.

"We also ask that parents speak to their children about being mindful about what they post and share on social media.

"We ask that anyone with direct information about a potential threat or post to contact our investigators at (281) 634-5500. As a reminder, students can remain anonymous by contacting FBISD Crime Stoppers at (281) 491-8477. Parents and students may also send an anonymous tip through text messaging by typing GETPAID, followed by their message, and sending to 274637 (CRIMES).

"Thank you for your continued support and partnering with us to keep our schools safe."

Another social media page named seven schools in southeast Texas, apparently in connection with another threat, calling those locations "main targets." Police said they do not believe that the threat is credible.