LOS ANGELES (CBSLA.com) — The lone gunman responsible for the Orlando massacre was described by co-workers as unhinged and unstable.

They said he spoke often of killing, and his ex-wife described Omar Mateen as disturbed and physically abusive – a profile of a serial killer chillingly reminiscent of others who went before him.

READ MORE: SoFi Stadium Looking To Fill More Than 3,000 Part-Time Positions With Job Fairs Being Held This Month

In 2009, Fort Hood army psychologist, Nidal Hasan, killed 13 people and injured 30. He was described as socially isolated, disconnected, aloof and paranoid.

Three years later, James Holmes murdered 12 people and injured 70 others inside a movie theater in Aurora, Colorado. In his notebook, he wrote about an obsession to kill 10 years prior to the attack.

The 3 lone wolves shared similar behavior. They were angry, isolated and committed to killing.

READ MORE: Feds: Mustafa Qadiri Of Irvine Used $5 Million In Fraudulent PPP Loans To Buy Ferrari, Bentley, And Lamborghini

Deputy Chief Michael Downing of the Los Angeles Police Department’s Counter-Terrorism and Special Operations Bureau said law enforcement cannot be everywhere all the time. So he asked the public to keep and eye out for suspicious behavior and activity.

“The message is that there’s only a limited amount of law enforcement and FBI, and that we really need the community to really step up,” said Downing.

He said if you are aware of someone who expresses hatred and advocates violence, pay attention to see if they plan on making their thoughts a reality, like planning an attack or acquiring weapons.

“It’s anything that targets innocent people with violence is we want the public to look out for,” Downing added.

MORE NEWS: 2 More Drivers Report Windows Being Shattered On SoCal Freeways

He said there are 10,000 sworn police officers in Los Angeles. He is asking for help from the 4 million people who live in the city.