A six-hour lockdown at Naval Medical Center San Diego (NMCSD) has been lifted after all personnel and patients sheltered in place Thursday following the report of an armed man at the hospital.

The situation prompted the naval hospital to issue a "code white" alert across the campus as naval officials launched an investigation. Several hours into the lockdown, no suspect or weapon had been found, officials said.

Brian O'Rourke, media relations officer for Navy Region Southwest, told NBC 7 the shelter in place order was issued as a precaution. There was no active shooter situation at the Naval facility.

At 12:20 p.m., Naval Base San Diego officials confirmed that four unspent bullets had been discovered during security sweeps of the medical center. Investigators were still working to determine whether those bullets were connected to the anonymous tipster's phone call.

The unpsent bullets were all found in Building 1, scattered in different spots in the building. At that point, the lockdown remained in place. By 1:40 p.m., the shelter in place order had been lifted and people were once again allowed to walk freely around the facility.

4 unspent bullets were discovered during security sweeps of the Naval Medical Center. Investigation is ongoing to... https://t.co/iyOENYnhQy — NBSD (@NavBaseSD) March 24, 2016

[G] Naval Medical Center San Diego on Lockdown

O'Rourke said an anonymous call came into NMCSD at 7:30 a.m. reporting an armed man on the fourth floor of Building 1 in the main hospital building. The caller then quickly hung up the phone.

Officials worked for hours to verify the call and determine whether there was a credible threat to the Naval facility or if this incident was a false alarm.

O'Rourke said the Navy takes any and all potential threats seriously. Right away, officials were deployed into the area and the facility was put on lockdown.

Brian O’Rourke, media relations officer for Navy Region Southwest, speaks with NBC 7 about the lockdown at Naval Medical Center San Diego on March 24, 2016.

“Our investigators have been thoroughly investigating that building since, we have not yet found anything. That investigation continues - floor-by-floor, wing-by-wing,” O'Rourke said around 10:10 a.m.

"Any threat to our people, to our assets, we take very seriously," he added. "We have to; we're going to make sure that our people are safe, that our neighbors are safe."

Naval Medical Center San Diego (NMCSD) was placed on lockdown Thursday as officials investigated a report of an armed man allegedly seen at the facility. This raw ground video shows police officers turning away staffers and patients at the entrance.

NMCSD officials advised staff and patients en route to the facility not to head to the hospital until the scene was cleared. People already at the campus would not be allowed to leave until officials gave the green light.

O'Rourke said patients who had appointments scheduled at NMCSD today will have those appointments rescheduled.

"If you need emergency care, there are other hospitals you can go to. Please call 911," he advised. "You can go to one of the clinics on one of our bases or any other hospital."

****** CODE WHITE ******A staff member reported seeing an armed man on 4-West. No shots have been fired. Security is... Posted by Naval Medical Center San Diego (NMCSD) on Thursday, March 24, 2016

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Aerial footage of the facility around 8:15 a.m. showed guards at the gates turning away vehicles. Officers with the San Diego Police Department (SDPD) were stationed at the entrance to NMCSD on Florida Drive as well.

Commander Navy Region Southwest Police patrol cars were parked on the campus of the medical center, with several uniformed officers also in place. Some ambulances and San Diego Fire-Rescue Department (SDFD) engines surrounded the complex, too.

At around 8:45 a.m., a pregnant woman at the entrance to the hospital was loaded into an ambulance. SDFD spokesman Lee Swanson said the woman was going into labor but couldn't enter the Naval Medical Center due to the lockdown.

Witnesses said pregnant woman was standing on Park Boulevard, just outside the facility, with many others when she collapsed. Swanson said the woman was transported to Scripps Mercy Hospital. She was rubbing her stomach as the ambulance took her away, trying to breathe through her contractions.

Throughout the lengthy lockdown, NBC 7 spoke with people waiting outside the facility. One man said his wife and child were inside NMCSD, hiding in a closet, waiting for officials to clear their building.

Kara Nietbalski told NBC 7 she drove two hours with her family, including her baby and child, for medical appointments at the center. She was waiting to get in.

"I'm scared for the people. I hope it's a false alarm. At the same time, if it's a false alarm, I'm frustrated because I drove two hours [for my baby's] appointment and now we're waiting," Nierbalski said, rocking her infant.

Just after 10:30 a.m., Naval Base San Diego posted a update to its Twitter page saying investigators were still sweeping buildings at the site:

Teams are still clearing buildings at the Naval Medical Center. We will continue to provide updates as more... https://t.co/PaoU00w2mh — NBSD (@NavBaseSD) March 24, 2016

As of 12:20 p.m., officials were still sweeping Building 1, Building 2 and Building 3.

About an hour-and-a-half later, the facility was all clear and people at the Naval hospital were once again allowed to move around freely. Naval Base San Diego officials said the hospital could begin resuming its normal operations.

Building One sweep is 100% complete. The shelter in place has been lifted. #NavalMedicalCenter... https://t.co/z6ZkQaBwLb — NBSD (@NavBaseSD) March 24, 2016

The large staff at NMCSD is comprised of more than 6,500 military, civilian, contractor and volunteer personnel. The 272-bed, multispecialty hospital and ambulatory complex is located on 78.4 acres in the southeast corner of Balboa Park, at 34800 Bob Wilson Dr., about 2.5 miles from downtown San Diego.

NMCSD personnel deploy to Iraq, Afghanistan, Kuwait, Djibouti and aboard the USNS Mercy. The facility is affiliated with 19 civilian nursing schools, training more than 400 students per year in clinical rotations.

The military hospital treats families of military members, too.

More than 250,000 San Diego residents are eligible for care at NMCSD. On an average day, it admits 45 new patients and handles 50 operating room cases, 170 emergency room visits and 4,000 outpatient visits. Eleven primary-care and 10 dental clinics at the site offer care to active duty service personnel and their families.

Almost exactly two months ago -- on Jan. 26 -- a similar situation led to a lockdown at the Naval hospital.

In that instance, NMCSD was placed on lockdown and people inside were ordered to shelter in place after an unidentified Department of Defense employee reported hearing three shots fired in the basement of Building 26. Officials swept the grounds and ultimately found nothing indicating a threat to the Naval hospital. No one was hurt in the Jan. 26 incident.

