Joint Base Pearl Harbor-Hickam implemented a nearly three-hour lockdown Tuesday due to a suspected bomb threat, but the gates were reopened after security personnel swept the installation and found no explosives, officials said.

The incident began at 10:30 a.m. when base occupants were ordered to “shelter in place” and limit traffic due to a “potential security incident,” causing vehicles to be backed up in the industrial area on the western edge of Honolulu.

The announcement of the lockdown also caused confusion for some base residents and community members because it coincided with the start of a pre-planned annual counterterrorism exercise called “Citadel Protect”– which is scheduled to run until June 17.

All tour operations to the USS Arizona Memorial also were suspended during the lockdown. Guests at both the USS Missouri Memorial and the Pearl Harbor Aviation Museum on Ford Island sheltered in place as well. All historic sites have since reopened after the all-clear was issued at about 2 p.m.

An official familiar with the incident, who spoke on condition of anonymity in exchange for providing details, said that a suspected bomb threat initiated the lockdown and investigation, but that personnel had swept the base and no bombs were found.

The joint base is considered one of America’s most important military installations, supporting Navy and Air Force operations across the Pacific region. It’s also home to the Pearl Harbor Naval Shipyard, the state’s top industrial employer. The base has a population of roughly 55,000 troops, military dependents and civilian employees.

A Monday press release about the exercise Citadel Protect said that “base personnel, residents and those who live near the installation should not be alarmed by the sounds of gunfire, other popping noises and activations of the base-wide Giant Voice System” and that “residents and personnel who live and work on JB PHH may see increased traffic and delays in base access.”

But base officials stressed that the lockdown and concurrent investigation were not part of the exercise. Navy Region Hawaii spokeswoman Lydia Robertson said Navy security personnel put the exercise on hold and immediately directed their effort toward the investigation and looking for potential threats.

Units from the Honolulu Police Department and Federal Fire Department also assisted in the investigation.

“We are grateful to the first responders who helped us investigate and secure the potential incident,” Col. Tammie Harris, Deputy Joint Base Commander, said in a press release. “I want to assure everyone that we will do everything possible to maintain the operational readiness of JBPHH while protecting the safety of our base employees, our service members and their families.”

The base also went into a lockdown on May 26, but that was also lifted after the threat was investigated. Officials did not explain specifically what prompted that lockdown.