The deadly shooting at the Islamic Center of San Diego has left southern California's Muslim community reeling, with grieving residents directing their fury at city leaders for ignoring years of warnings over rising Islamophobia.

Two gunmen opened fire outside the mosque shortly before midday prayers on Monday, killing Amin Abdullah, 51, Nadir Awad, 57, and Mansour Kaziha, 78, known affectionately in the community as Abu Ezz.

Police and the FBI are investigating the shooting as a hate crime, after discovering extremist writings linked to the attackers.

Authorities say the pair appeared to have been influenced by neo-Nazi propaganda and previous anti-Muslim attacks, including the 2019 Christchurch mosque massacre in New Zealand.

For many in San Diego's Muslim community, the attack has deepened fears over rising Islamophobia in the United States.