​A gunman killed 12 people, including a sheriff's deputy, late Wednesday night when he opened fire on revelers in the Borderline Bar and Grill, a popular venue with college students for dancing in the suburb of Thousand Oaks, California. Here's what we know, according to various news reports:

The gunman was identified as Ian David Long, a 28-year-old veteran of the U.S. Marine Corps who had previous run-ins with the law.

The gunman was dressed in black and wearing glasses. He walked up to the Borderline Bar & Grill and shot a security guard standing outside. He then entered the bar, turned right and shot other security guards and employees.

Sgt. Ron Helus was shot several times and killed during an exchange of gunfire with the gunman. Helus and a California Highway Patrol officer were the first to arrive at the bar and went inside just before 11:30 p.m. PT.

The gunman was armed with a Glock .45 handgun, purchased legally, and some type of smoke device.  The gun usually holds 10 rounds, but the gunman used an extended magazine in this shooting.

The shooter's motive is unclear, as is any possible connection between Long and the Borderline bar.

Police had several contacts with Long previously. In April, officers responded to a disturbance at Long's home. Long was somewhat irate and acting irrationally. A mental health specialist met with him and ultimately decided not to detain him.​

The gunman was on active duty with the Marine Corps from August 2008 to March 2013.

 It was the fourth shooting in the United States in two weeks. The attacks include the death of two women at a yoga class in Tallahassee, Florida, and two shoppers gunned down in the parking lot and inside a Kroger in Jefferson, Kentucky. Twelve days ago, a gunman killed 11 worshippers at a synagogue in Pittsburgh.​

President Trump on Thursday ordered flags to be flown at half-staff in remembrance of the 12 people.

Recent ASIS resources on Soft Targets:

Soft Targets, Hard Challenges​

Soft Targets Q. and A. 

Harden Soft Targets with PSIM​

Lessons in Violence Prevention