"These began shortly after the death of his mother when he was 10 or 11, and have become progressively worse," Brennan said.
Brennan says Rivera had a juvenile arrest record dating to 2001 and had been arrested for driving without the owner's consent, grand theft and assault with a firearm. He was convicted of the latter charge and had served probation.
There was no evidence that Rivera was a gang member, but police had been called to the house five times in one year for minor disturbances. Brennan said there was also a drive-by shooting at the house in August 2006, but no one was injured.
More than 24 hours after the night-long standoff came to an end, dozens of officers remain on scene, collecting evidence and trying to determine what led to the deadly shootout.
They're looking into a number of things, including the ballistics and the rapid deployment technique that was used by SWAT officers when they first entered the home.
There are still countless questions about the young gunman and what triggered him to shoot five people, including close members of his own family. The coroner's officer has confirmed the gunman, who was killed by police, is 20-year-old Edwin Rivera.
A friend of his told Eyewitness News that Rivera was a high school dropout and at times he would act and say things that were strange.
"We'd be like hanging out or something, talking about a certain thing and he would just say something random, like, something that was in his mind," said Jose Ortiz, Edwin's friend. "Everybody would just awkwardly pause and look, and just keep on talking."
Police first responded to the Rivera home on the 19800 block of Welby Way Wednesday night after a caller, who they believe was Edwin, called 911 and said he had killed three people and 'to come and get them.' Edwin's victims were members of his own family.
They have been identified as his 54-year-old father Gerardo Rivera, an immigrant of El Salvador, and his 21-year old brother Edgar. The other brother hasn't been officially identified, but friends say his name is Andy Rivera. He's believed to be 25.
When the SWAT team got to the scene, they didn't know for sure if the Rivera men were dead. They also had information that lead them to believe that there may be more people inside of the home, possibly being held hostage. That's when they made the decision to move in.
It was a decision that would ultimately cost veteran police officer Randal Simmons his life, and would critically wound officer James Veenstra.
"Their intention in going to this location was to enter the location and rescue any of the victims that were inside that might still be alive," said Deputy Chief Michael Moore.
At 12:30 a.m. Thursday, SWAT officers entered the one-story home. Upon entering the home gunfire immediately erupted and the two veteran SWAT officers were shot. Police then retreated from the home.
It was firefighters that ran into the house to rescue those fallen officers and one of the Rivera men that at the time still showed signs of life.
Both officers were taken to Northridge Medical Center, where one of them, Randal Simmons, 51, died just after 1 a.m. He was a 27-year veteran of the force. A source within the LAPD told Eyewitness News that Simmons was shot in the neck and that bullet lodged in his brainstem.
The other officer, 51-year-old James Veenstra, was shot in the face and critically injured. He underwent a three-hour surgery Thursday and is in stable condition.
Hours later, additional tactics were used to get Edwin Rivera to surrender. Shortly after 6 a.m., police lobbed tear gas into the home.
Although Rivera did not surrender and stayed inside, a woman came running out. The woman was reportedly the girlfriend of Edwin's father, Gerardo Rivera. Published reports say that relationship caused friction within the family.
Eleven hours after the suspect barricaded himself inside the home, the standoff came to end when a diversionary device known as a flash-bang sparked a fire. Edwin was then forced to flee the home and was reportedly shot in the head by a sniper.
The one woman who escaped from the home may be key in giving investigators some information as to why Edwin Rivera started shooting.
The investigation continues on Welby Way. Officers will work through the day and possibly into Saturday.
The road remains blocked off. The only way residents can get in is with a police escort.
Trust funds have been established through the Los Angeles Police Federal Credit Union for Officer Randal Simmons and Officer James Veenstra.
If you'd like to make a donation to the fund click here. When on the site (www.lapfcu.org) just click on the red square titled "LAPFCU Community Corner." You can also make a donation by contacting the Los Angeles Police Federal Credit Union at (877) 695-2732.
Eyewitness News reporter Wendy Burch and The Associated Press contributed to this report.