The three- to four-page note was found on his body after he died. He also said goodbye to friends and expressed hope his mother would be taken care of.
Police also said the Santa Monica house fire on Yorkshire Avenue where his father and brother were found shot dead was intentionally set before Friday's shooting rampage.
A preliminary police investigation reveals the house fire is believed to be intentionally set in two different locations inside the house.
Suspected gunman John Zawahri, 23, is believed to have set his father's house on fire Friday, then carjacked a woman on the street after firing several shots from a semi-automatic rifle, before proceeding to Santa Monica College, where he continued shooting. He was shot in a firefight with police in the school's library and died on the sidewalk outside. Four people were killed by Zawahri.
Two officials briefed on the investigation said the semi-automatic rifle Zawahri used appears to have been built with component parts that are legal to obtain, but put together make the rifle illegal in California.
In Zawahri's bedroom, investigators found a drill press among other materials that indicate he likely assembled the weapon.
The drill press is used to help finish building the rifle by drilling holes in the lower receiver. A lower receiver that is only 80 percent complete can easily be purchased, and because it is not complete, a person isn't required to go through a background check, nor does the part need to have a serial number.
In California such weapons require a "bullet button" kit, which needs to be added to a lower parts kit to make it legal. The bullet button kit modifies the weapon so that a separate tool must be used to release an ammunition magazine and reload the gun; without such a modification a person can press a button to release the magazine.
According to burn patterns in the house, video and eyewitness accounts, the point of origin of the house fire on Yorkshire Avenue was in the front living room.
A second fire is believed to have been set in one of the bedrooms. One of the beds was determined to be a second point of origin.
Matchboxes were found in the bedroom. It was not clear if matches were used to set the blaze.
Two men, the father and brother of shooting suspect John Zawahri, were found dead in another bedroom in the house.
It was not immediately determined if the suspect used an accelerant. Materials were being laboratory-tested.
Zawahri was carrying 1,300 rounds of ammunition in magazines that were capable of holding 30 rounds each. Such high-capacity magazines are illegal to purchase, sell or transfer in California. Possession is not illegal.
Zawahri's last reported contact with law enforcement was seven years ago, when bomb-making materials were found at his house during a search prompted by threats to students, teachers and campus police officers at Olympic High, a school for students with academic or disciplinary issues.
Retired police officer Cristina Coria, who helped serve the search warrant, said Zawahri was hospitalized for psychiatric evaluation at the time. She didn't know the outcome of the evaluation.
Police declined to provide further details, saying Zawahri was a minor at the time. But once a person is held for such an exam, they cannot access or possess firearms for five years.
In the case of Zawahri, that prohibition would have expired in 2011.
The investigation was ongoing.
The Associated Press contributed to this report.