POST FALLS - Three similar calls to schools in the region in five days, all warning of gun violence, have police agencies collaborating on tracking the source.

The calls were made to Ponderosa Elementary in Post Falls and Farmin Stidwell Elementary in Sandpoint on Thursday and to Northwood Middle School in Spokane on Monday.

In all of the cases, the calls were made between 2 and 2:30 p.m. and described by police as an electronic recording. All of the schools were locked down and police made methodical sweeps of the sites, but no intruders or suspicious items were found.

Post Falls Police Chief Scot Haug said his agency has obtained a series of phone numbers that made calls to Ponderosa on Thursday afternoon and is in the process of pinpointing the source that made the threat.

"When we have the telephone number, we will seek a search warrant to obtain the owner's identification," Haug said. "We're taking this case very seriously and it's a high priority to us. It doesn't get much more serious than threats to children. That's why we sent all of our available resources."

Haug said the investigation may take as long as a week, but his agency will collaborate with police in Sandpoint and Spokane due to the calls being similar.

"Contrary to what some people think, we don't have immediate access to phone records," Haug said. "We need to work with the phone companies to track down numbers. It can take some time to track down."

Haug said had there been an actual incident, police likely could have obtained phone records sooner.

Haug said he remains hopeful that the source of the call or calls can be found, but it's far from a slam dunk.

"In this day of technology, people can spoof their numbers and always use public phones," he said. "It certainly can be a challenge."