A tip regarding plans for a shooting on Yale’s campus Monday brought SWAT teams to the University and forced a five-hour-plus lockdown, which ended only when New Haven and Yale Police Department officials said they could be confident that the threat had either passed or never existed.

At 9:48 a.m. on Monday, the NHPD received a call from an anonymous man who said that his roommate had a gun and was headed to the University’s campus with plans to shoot people, according to NHPD spokesman David Hartman. Hartman said that authorities were not able to extract further information from the caller, who hung up just seconds after placing the call from a phone booth in the 300 block on Columbus Avenue. At 10:50 a.m., the Yale Alert System mandated a campus-wide lockdown via email, phone call and text message to students, faculty and staff. Updates continued throughout the day.

“Confirmed report of person with a gun on/near Old Campus,” said a Yale Alert text message sent to the University community at 11:02 a.m. “SHELTER IN PLACE. This is NOT a test.”

NHPD and YPD officers were joined on campus by four SWAT teams and agents from federal law enforcement agencies including the Federal Bureau of Investigation. To facilitate the investigative sweep of campus, major city streets were shut down, particularly around the intersection of Elm and College Streets. Early reports indicated that a gunman might be located in or near Battell Chapel, which is located at that intersection.

Police conducted a series of room-to-room searches, which began in Calhoun College at around 1:45 p.m. NHPD Chief Dean Esserman said that searches would continue in all Old Campus rooms before a definite conclusion on the matter could be reached. No injuries or gunfire were reported at any point throughout the day, and the lockdown was lifted in all campus areas at 4:5o p.m.

“Once we have geared up, we don’t just gear down,” Esserman said at a press conference before the University had ended the lockdown. “That is why we’re going back [to Old Campus] to stay the course. We have a very strong operations plan where we are going room to room … until we make sure that we are satisfied.”

As officers from the NHPD and YPD were initially dispatched to the scene, a Yale employee reported seeing a man walking around campus with a rifle. That this second tip came in after the University lockdown had started seemed to legitimize the first phone call received by police in the morning. Esserman later revealed, however, that the witness might have mistaken an armed police officer for a gunman.

Though the investigation has not been officially closed, a lack of concrete leads since the early morning has led Esserman to conclude that there is a possibility that the phone call received at the beginning of the day was disingenuous, he said.

University Vice President Linda Lorimer sent an email to all University faculty, staff and students to review the day’s events.

“The combined police forces combed the area and have found no suspicious person,” Lorimer said in the email. “However, the police are taking nothing for granted. They are working to track down who made the first (anonymous) phone call …  if it was a prank call that started this chain of events, the authorities intend to prosecute the individual to the fullest extent of the law.”

The episode unraveled on a relatively empty campus, as many students have vacated their dorms with the start of Thanksgiving break this weekend. Those that remained on campus, however, were forced to take precautionary measures as part of the lockdown. Some residential college masters emailed students advising them on how to deal with the increased police force and threat of a potential gunman by telling them to lock their rooms and refuse entry to anyone, even those identifying themselves as police.

When the plan to search room by room was announced, YPD Chief Ronnell Higgins sent an email to students notifying them of the procedure that would follow. Instructions were also delivered over the Yale Alert system.

“Out of an abundance of caution, Police will be doing a room to room search starting with the residential college areas,” Higgins said in the email. “When they knock on your door, a Yale Police Officer will slip their Yale ID under the door. Please cooperate.”

Despite the mass influx of heavily armed officers, those present said the scene on campus was relatively stable. Rob O’Gara ’16 said that students around his suite in Saybrook were “very calm” as the lockdown unfolded.

Calhoun College in particular was the site of a large SWAT team effort —  a large, armored truck was stationed outsides its gates on Elm Street, and students were evacuated from their rooms to the college dining hall as in-room searches took place. Still, the focus of the investigation remains on the initial phone call that set the day into motion.

“You don’t just walk away … no matter how long it takes,” Esserman said. “We are going to find who [made the phone call] and we’re going to put handcuffs on who did this.”

Press conferences were held in the lobby of the Shubert Theater on College Street.

Tasnim Elboute contributed reporting.