Maryland:
>2019/02/04, Employee shot at University of Maryland hospital in Baltimore; suspect charged with attempted murder,
http://archive.is/j8vAj

>2010/09/16, Physician shot and wounded on eighth floor of John Hopkins Nelson building at 11:11,
http://web.archive.org/web/20100926103338/https://www.hopkinsmedicine.org/news/media/releases/update_on_shooting_at_the_johns_hopkins_hospital

>2019/02/04, Employee shot at University of Maryland Medical Center,
http://web.archive.org/web/20190206222935/https://wjla.com/news/local/breaking-news-shooting-reported-at-university-of-maryland-hospital

>2015/07/06, Shots fired inside Walter Reed Military Medical Center,
http://web.archive.org/web/20190220050556/http://www.fox5dc.com/news/allclear-no-evidence-of-shots-fired-inside-walter-reed-medical-center

>2018/11/27, Improper use of alert system led to active shooter reports at walter reed medical center,
http://web.archive.org/web/20181128031431/https://www.wusa9.com/article/news/local/maryland/improper-use-of-alert-system-led-to-active-shooter-reports-at-walter-reed-hospital-navy-says/65-618071853


Take note of the last two listed.
>2015/07/06
>ALL-CLEAR: No evidence of shots fired inside Walter Reed Medical Center
>There was a massive police response Monday on the grounds of Walter Reed National Military Medical Center in Bethesda after a report of shots fired at the facility was called in. It led to a campus lockdown and an order for those inside Walter Reed to shelter in place.
>But after hours of searching, police say they found no evidence that any shooting occurred and the all-clear came just after 2 p.m.

>2018/11/27
>BETHESDA, Md. -- Reports of an active shooter Tuesday afternoon at Walter Reed National Military Medical Center were unfounded and the U.S. Navy said Tuesday evening that "improper use" of the alert system led to an active shooter alert being sent out.
>This was a (((drill)))
>After the initial report, around 2:15 pm the Pentagon said the incident was just a drill. Around 4:20 p.m., the Naval Support Activity Bethesda (NSAB) said the report of an active shooter was a false alarm and not part of a scheduled drill. After a call came into security around 2 p.m., NSAB said security found no indication of an active shooter.

According to the U.S. Navy, it was people who saw the alert that notified security.

Multiple sources said all patients and staff were moved to secure locations and were advised to remain on lockdown until notified by security, who instituted an installation-wide active shooter response.