Editor's Note: The officer's identity has been released by the Medical Examiner.
A large police presence with more than a dozen Chicago Police Department squad cars lined the street in front of Northwestern Memorial Hospital downtown early Thursday after an off-duty officer inside the 18th District police station died overnight.
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According to a police source, the female officer died after a self-inflicted gunshot wound. A death investigation was underway, police said.
"The Chicago Police Department is mourning the tragic loss of an officer within the 18th District station," CPD said in a statement. "Our officers are human, and we cannot forget that they face the same challenges as everyone else while balancing the difficulties that come with being a police officer."
"We ask the public to keep this officer’s family and her fellow officers in your prayers as we grieve this unimaginable loss," the statement went on to say.
The 18th District station is closed for service as the investigation continues, police said. Service calls within the district are being handled by nearby districts.
Around 2 a.m., an ambulance was called to the station, located at 1160 N. Larrabee St. on the Near North Side, officials said. The station was closed off for hours, NBC 5's Sandra Torres reported, with streets near the station reopening to traffic around 5 a.m.

Photos and video overnight from the scene showed emergency crews carrying a stretcher out of the station, with police officers lining the sidewalk in salute.
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"Usually, this is an indication this person is a member of the Chicago Police Department," NBC 5 Sandra Torres reported Thursday morning from the station.
Torres added that the medical examiner was also on scene.
Around 5:30 a.m., NBC 5 photographer George Mycyk captured nearly a dozen police cars outside of Northwestern Memorial Hospital in Streeterville, with some streets near the hospital blocked off.

A procession took place Thursday morning from the hospital to the Medical Examiners office.
In a statement, Chicago Mayor Brandon Johnson called the loss "tragic."
"Our hearts are shattered and broken over this tragedy," Johnson's statement said. "My deepest condolences and ardent prayers are with the officer’s family, loved ones, and fellow officers as we grieve."
"This tragedy is a reminder that we must carry each other, the statement went on to say. "Every single day, our
officers witness traumatic scenes, and it is our responsibility to provide them with the support they need to navigate that reality. I ask the city to keep the officer’s family and loved ones, 18th District officers and staff, and the entire Chicago Police Department in your prayers during this time of immense grief.”
If you or someone you know is having thoughts of suicide, call or text 988 for the Suicide and Crisis Lifeline, or contact the Crisis Text Line by texting HOME to 741741.