Students gave Richard Martinez a standing ovation after he shared his pain and his plan to send leaders postcards with the message "Not one more."
The father who lost his only child, Christopher Ross Michaels-Martinez, asked students to use the hash tag he just learned about to spread the word.
Many students chanted the words, while a few worried it would be too political and take the focus of mourning.
Richard also read remarks from the parents of other victims who said they wanted to pray for the killer. Another family said his parents lost a child too.
For the first time since the murder of six students, the UCSB community heard from Chancellor Henry T. Yang and University of California President Janet Napolitano.
The ceremony ended with an A capella performance and an interfaith prayer.