The vegan bodybuilder based in San Diego, Southern California travelled to YouTube's headquarters and opened fire on a 36-year-old man said to be in a critical condition, and two women aged 32 and 27 before shooting herself.
Aghdam's father, Ismail, told local US media that she was angry because YouTube had stopped paying her for videos.
Ismail reported his daughter missing on Monday after not answering calls for two days. Police later found her sleeping in her car in Mountain View, 25km from YouTube HQ in San Bruno and reported this to her family.
Her father told police she might go to YouTube as she "hated the company" but she was not detained at the time, local media reported.
NBC has since found a video the assailant posted to her YouTube channel in 2017 where she accused the popular streaming service of hiding her content from her followers.
In the video, Aghdam claims her channel stopped receiving traffic after YouTube started to filter her content and therefore she was making less money.
Her channel consisted of workout tutorials, discussions about animal cruelty and angry videos where she rallied against YouTube.
In one video, she quotes Adolf Hitler, saying: "Make the lie big, make it simple, keep saying it, and eventually they will believe it."
She also wrote: "There is no equal growth opportunity on YouTube or any other video sharing site. Your channel will grow if they want [it] to!"
YouTube has since terminated her account and her Facebook and Instagram pages have also been taken down.
If you or someone you know needs help, you can speak confidentially to a trained counsellor 24 hours a day at headspace.org.au or Kids Helpline: 1800 55 1800.