Witnesses said what was supposed to be a festive event quickly turned into a terrifying experience.
The annual carnival at St. John de la Salle Catholic Church and School was just getting underway around 11 a.m. when Diaz appeared.
"I saw him come in with a tennis bag, and I saw him go to the end, and I saw him take the gun out," said Kassandra Reyes, one of the many witnesses at the carnival.
Authorities said Diaz had an ongoing dispute with his 30-year-old ex-wife. There were dozens of parents and children gathered at the carnival, including a boy believed to be his son.
"He came, and he kissed his son, and he pulled out an assault rifle and shot three people," said witness Martinique Williams.
The Diaz appeared with a rifle, a semi-auto .22 caliber rifle, and began to fire in the direction of his ex-wife. During the course of the shooting, the ex-wife was shot in the arm," said LAPD Capt. Steven Ruiz.
A 45-year-old man was also shot in the leg, and another 47-year-old man was shot in the chest. Both were taken to local hospitals and are expected to survive.
After firing the shots, Diaz ran but was chased down by several people attending the carnival.
"I did an excellent open-field tackle," said local resident Charles Sternberg.
"I just tackled him as hard as I could and put him on the ground and then everybody else jumped on him," Sterberg said.
An off-duty LAPD officer also helped subdue the suspect and take him into custody.
Diaz was booked at Valley Jail in Van Nuys, with bail set at $1.5 million. He faces three counts of attempted murder and possibly more charges.