Meanwhile, the company released a statement Wednesday saying the "shocking and disturbing" event will lead to more security at its offices worldwide.
YouTube also praised San Bruno police for an "exceptional" response after being called for help and hailed "numerous acts of heroism" to employees and police officers that were on the scene.
Family members first reported Aghdam missing on Monday, filing an "at-risk" report with the San Diego County Sheriff's Office as she'd recently lived with her grandmother in the area.
Mendoza says he did not understand if Aghdam bought ammunition or accessories.
But her brother said he expressed additional concern upon learning she was discovered by police so close to the YouTube campus.
The woman shot three people at YouTube's headquarters before killing herself. They called her family to report that they had found her after. The police say that they didn't hear anything about that and they spoke with her for 20 minutes and didn't notice a gun or anything of the sort.
The attack in San Bruno marks the latest in the series of wide-spread gun violence in the US.
Nasim Aghdam opened fire with a handgun and wounded three people at YouTube's headquarters, before she killed herself.
Aghdam's father, Ismail Aghdam, said he told police his daughter might go to YouTube's headquarters because she hated the company, but police disputed that account.
The officers arrived Wednesday afternoon after two police investigators and one officer from the nearby San Bruno police spent almost two hours at the Jackson Arms Shooting Range in South San Francisco. She then headed to a local gun range on Tuesday before traveling to YouTube HQ and executed the firing during lunch time. Now, remember, this happened in an open courtyard at YouTube's headquarters during lunch.
Witnesses reported helicopters on the scene as well as police SWAT teams. He left almost two hours later without speaking with reporters.
Spokeswoman Ginger Colbrun would not confirm the locations but reporters on Wednesday saw agents entering homes in the communities of Menifee and 4S Ranch. Police found her early Tuesday morning asleep in a parking lot in Mountain View, Calif., just a few hours before the shooting. In an interview with CBS Los Angeles, her father said she continued to harbor ill will toward the video platform, and that she had accused it of "ruining her life".
"It's not like she stood out", said the store's rangemaster and retired officer Manny Mendoza.
"We know that she was upset with YouTube and we've determined that right now that's the motivation that we've identified", Barberini said.
Aghdam did not have any ties with any of the shooting [VIDEO] victims and police were investigating two homes that were associated with the suspects.
Officers in Mountain View - about 30 miles (48 kilometers) from YouTube's headquarters - found her sleeping in her vehicle in a parking lot around 2 a.m. Tuesday but let her go after she refused to answer their questions.
The car's license plate matched a missing person's report from her family. "I mean maybe not today, but she never hurt any creature", he said.
Police said in a statement that "she in no way met any reason for us to speak with her further or possibly detain her".
Those channels, along with Aghdam's website and profiles on Facebook and Instagram, had all been taken down by Wednesday afternoon.
As of right now, the three shooting victims were rushed to Zuckerberg San Francisco General Hospital. The third shooting victim, a 36-year-old man, was upgraded from critical to serious condition.
In the event of the attack, Aghdam fired multiple shots from a Smith and Wesson semiautomatic handgun which was registered in her name. She appears to have been motivated by her anger at YouTube. She gave no indication she was a threat to herself or others.
He says 39-year-old Nasim Najafi Aghdam got into the building through a parking garage. "There may be controlled access to get into that work space", Cohen said, but "that controlled access can be bypassed by someone who is really committed to do it". On Wednesday, investigators were conducting searches at two properties, San Bruno Police Chief Ed Barberini said.
Her father said that they had no idea about where she got the gun from. She also complained that YouTube was discriminating against her and is not monetizing her video content.
Barberini said the injured "have been transported and are being treated for injuries that are treatable".
"They remained calm throughout this second phone call", police said. But a law enforcement official said Aghdam had a longstanding dispute with the company. The officials spoke on the condition of anonymity because they were not authorized to discuss the case.