"We have been encouraging our mosque communities to take security precautions," said Ahmet Tekelioglu, executive director of CAIR Philadelphia.
He added, "You always want your mosque, your church, your synagogue to be a welcoming place, not a fortified securitized kind of place, but at the same time these incidents are all too real."
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Police said two suspects, ages 17 and 18, were later found dead from self-inflicted gunshot wounds several blocks away. One of the suspects had been reported missing earlier in the day by his mother, who told authorities her car and weapons were gone and that she feared he was suicidal.
"While we were gathering this information and we were trying to zero in on where these individuals were at, we received a call at approximately 11:43 that there was an active shooter at the Islamic center," San Diego Police Chief Scott Wahl said.
The victims include a shop owner at the mosque, a mosque security guard and the husband of a teacher at the mosque. Children were seen being escorted off campus hand in hand.
Authorities said no specific threat to the Islamic Center or any other location had been identified before the shooting. However, investigators found anti-Islamic writings in the suspects' vehicle, indicating possible hate-driven motives.
Philadelphia police took to social media in response on Monday, saying, "PPD is closely monitoring reports regarding the active shooter incident at the Islamic Center of San Diego. While there is currently no known threat to Philadelphia, out of an abundance of caution, the PPD is increasing patrols around mosques and other sensitive locations."
Local leaders are calling for unity in response to the violence.
"Continue to strengthen our ties, I think the best response to hate and violence is to continue to show the solidarity and broader shared humanity," Tekelioglu said.
The investigation remains ongoing.