Denton police are investigating two bomb threats mailed to two Denton churches this week.

Officials at St. Andrew Presbyterian Church in Downtown Denton reported receiving a bomb threat Tuesday similar to one sent to Denton Bible Church Monday. Both churches said the letters threatened an attack during the Christmas Eve service.

"Any time that there's a threat of a bomb...there could be mass casualties and we want to take as many precautions as possible to prevent that from happening," Denton Police Department spokesman Shane Kizer said.

Police did not release any details about the investigation, but are taking the threat seriously as officers will patrol all churches in town. They said no religious or social affiliation was mentioned in the letters.

"The police department is working with staff from both churches in an effort to identify and locate the person or persons who issued the threats. The police department will also have a stronger police presence at and near our places of worship during the Christmas holidays," Kizer said in a statement Tuesday.

Both churches said they will hold Christmas Eve services despite the threat.

"They feel that is the right thing to do, especially with their mission statement," Kizer said. "And they are going to take all precautions necessary to make sure they are safe."

Church members plan to be there.

"This is all over America. You can either shut down America or just keep doing your thing and be vigilant," said Denton Bible Church member Bruce Wakefield.

Saint Andrews church leaders drafted a letter Tuesday to give to members, saying the congregation will not succumb to fear.

"Once it hit the news that this was going on, we felt it better to make a statement so that everyone is on the same page," said the Rev. Alan Baroody, interim pastor at Saint Andrews.

Baroody said it's vital for the community that Christmas Eve services go on. Sain Andrews will host services a 5 and 7 p.m.

"As Christ came to dwell with us, so we dwell with each other – witnessing to a love that is greater than fear, prejudice and hate," he said.

Still, Saint Andrews is stepping up security in light of the threat. Baroody said two Denton police officers will be stationed across the street all day on Thursday, long before services begin.

Police said they think one person mailed both bomb threats. Baroody said the church did not retrieve its mail Monday, so it's possible the threat was in their mailbox at the same time Denton Bible Church received its threat.

Police are still searching for the person who mailed the threats, because it could be "hate-related crime."

"These are both Evangelical churches, so it could be towards that," Kizer said. "Or it could be someone wanting to cause alarm or cause panic."

Police confirm that they've reached out and briefed the FBI and other federal investigators about the threats, but won't comment about their possible involvement in the investigation.

Police are also not saying if the letter was hand-written or typed.

"Both letters were similar in makeup," Kizer said. "It was a short letter. Just a few sentences. I'd rather not go into the specifics of the letter so not to interfere with the specifics of the investigation."

Online: Denton Bible Church | St. Andrew Presbyterian Church

NBC 5's Ellen Bryan, Brian Scott and Jeff Smith contributed to this report.