TERRE HAUTE, Ind. –– Five days later and with no suspects, police released the audio from a call that led to an hours-long lockdown at a Terre Haute high school in western Indiana on Friday.

“It’s frustrating that we have this huge incident and thousands of people were affected and unfortunately we don’t have anyone in custody yet,” Terre Haute Police Chief John Plasse said Monday of false threats that gripped Terre Haute North Vigo High School for hours Friday. “We’ll do everything we can to make [an arrest] happen.”

Authorities are looking into a possible connection between the incident and other hoaxes at schools elsewhere around the country, Plasse confirmed.

“That’s something we’re looking into with the other agencies involved, seeing if we can get a common thread or direction from them,” he said.

The suspect in the incident made two calls to the Terre Haute Police Department, Plasse said. One of the calls was transferred to Vigo County Central Dispatch where a 911 operator kept the man on the line for nearly 45 minutes.

In audio of the call released Monday, the suspect said he attended Terre Haute North Vigo High School, using the full name of the school, and twice pronounced the city as “Terre Hoot.”

Calling himself “Mike,” he said he had locked himself in a school bathroom and repeatedly warned that he and an accomplice, “Sam,” stationed at another location, would detonate pipe bombs and begin firing assault rifles if police “breached” the school.

Insisting the call was not a prank, he demanded $10,000 be placed in a duffel bag outside the school within 25 minutes or he would start shooting. He frequently reminded the dispatcher how much time remained. He later extended his deadline, only to accuse the dispatcher of “stalling” and said he and his accomplice would executive their plan.

But police searched the building and, aided by Indiana State Police bomb sniffing dogs, concluded there were no bombs and no one at the school with assault rifles.

Leslie Nees said her daughter, Haylee Higgins, 16, was in the school during the four-hour ordeal. Nees said the parents did not know for awhile what was happening inside the building.

“We were nervous and scared. I think police and everybody did what they needed to do. They couldn’t have chaos out here. It was hard, but you have to just pray.“

Four Terre Haute officers and a number of FBI agents are involved in the investigation, Plasse said. He indicated the origin of the call has not been traced.

“We’re still looking into who was responsible for making the call,” he said. “There are ways to block numbers or make them appear to be some other number. His true number wasn’t showing up on the caller ID.”

Plasse said he hopes the FBI has technology that can help identify the source of the call.

Police also welcome calls from anyone who may have knowledge of the incident, he said. Calls may be placed to the Terre Haute Police Department at 812-238-1661.

Taylor writes for the Terre Haute, Indiana Tribune-Star.