Long, a 28-year-old former Marine who likely had post-traumatic stress disorder, was carrying a Glock 21 .45-caliber handgun with an extended magazine.
Although the motive for the Thousand Oaks shootings is still unclear, officials said a mental health specialist had cleared Long from involuntary psychiatric commitment after the police were called in to investigate a disturbance at his house in April.
Such a commitment refers to the way in which a judge can order a person to have mental health treatment even if they do not want to undergo this.
U.S. President Donald Trump, who described Long as "a very sick puppy" and "a very, very mentally ill person", blamed mental illness for the deadly assault.
Ben Campbell, a survivor of Wednesday's shootings, said: "It's not a gun control problem. It's mental (health).
"They had what they needed, and didn't stop him," said Campbell, referring to the mental health evaluation the authorities conducted on Long.
Several high profile Democratic leaders, including Senator Elizabeth Warren of Massachusetts, have vowed to strengthen gun controls.
"This cannot continue," she wrote on her Twitter page, adding that Congress must act now to end gun violence.
Gavin Newsom, California's governor-elect, who will be sworn into office in January, urged people to take action against gun violence.
"Simply saying, 'enough is enough', is not enough," he wrote on Twitter. "We must address the root causes of these devastating acts at every level of government."
The debate ultimately comes down to gun ownership rights versus public safety.
The midterm elections on Tuesday resulted in some good news for gun control advocates. At least 17 Democrats newly elected to the House of Representatives have backed stricter gun laws, according to CNBC.
However, the Senate remained in the grip of Republican lawmakers, many of whom are supporters of Second Amendment Laws, which grant the right to bear arms.
The Thousand Oaks shootings, which took place just a day after the elections, prompted some locals to wonder if politicians are doing enough to address gun violence.
"I am angry because didn't we go through this two weeks ago?" said Sarah Silikula, a mother of eight who lives near the Borderline Bar and Grill.
"Didn't we just have elections? Why isn't this important?" she said. "We have a president that stands up to everybody, apparently. Why isn't he standing up to the guns? Why isn't he standing tall and saying 'enough is enough?' I dare him to do that."
The same question was on the mind of Isabella Robakowski, a senior at Newbury Park High School and president of NeverAgainSoCal, a grassroots organization formed after the Parkland High School shootings.
Robakowski said it was tough waking up to news of the latest incident on Thursday because she lives just over 6 km from the scene.
