An armed guard greeted students at University Park Elementary in Denver Monday morning and will remain at the school while threatening messages from a former school employee are investigated.

According to Denver Public Schools, parents requested more security at University Park after receiving a letter from the principal advising them of the threatening messages.

The former employee, whose name 9NEWS is not releasing because he has not been charged, began working for the school in 2015. He resigned as its facilities manager in September 2017.

According to a restraining order obtained by 9NEWS, the former employee sent text messages to the school's principal days later. In the messages, the former employee said he would "make sure justice is served," that the principal "would feel the power of [the former employee's] words," and that the principal would "pay for his mess in this life and the one after."

DPS spokesperson Will Jones said the messages came as a surprise since he says the former employee left on good terms.

"[The school] had a very good rapport with this man," Jones said.

After the initial text messages in October Jones says the principal contacted DPS's Department of

Public Safety who got in touch with the Denver Police Department.

He says no immediate actions were taken as the school has received messages from disgruntled employees before. That changed when another employee at University Park Elementary received threatening messages on Dec. 22 from the same former employee.

The messages to a teacher at the school said “that God... will serve justice," according to the restraining order.

Jones says that’s when the school began working with police to get the restraining order, which was granted on Jan. 10.

The restraining order states the former employee sent more messages in January to University Park's principal through email, saying he was "doing the devil's work."

Parents were briefed on the situation, and a community meeting was held Jan. 25.

Jones said the district granted the request from parents for more security at the school, assigning an armed guard from the district's Department of Public Safety to the school.

Jones said an armed presence will remain outside the school until they feel there is no longer a threat.

DPS is working with authorities and they believe the former employee is out of state.

According to the restraining order, the man’s former roommate said he would "sit in the dark and talk to angels and that he.... thinks he can kill people because the angels talk to him." The former roommate also indicated the man "wanted to get a gun."

Another community meeting with parents is scheduled for Thursday.