A gunman opened fire at an elementary school in the California city of San Bernardino, killing a teacher and an eight-year-old boy before turning the gun on himself, police said.
Another child caught in the crossfire was rushed to a hospital and said to be in critical condition.
“This does appear to have been a murder-suicide with both male adult and female adult victim succumbing to injuries, with the male succumbing to a self-inflicted gunshot wound,” Lieutenant Mike Madden of the San Bernardino Police Department told a news conference on Monday.
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“We had two students who were tragically injured and are listed as critical at this stage in area hospitals.”
Police later confirmed that one of the students, eight-year-old Jonathan Martinez, had died.
Police identified the gunman as local resident Cedric Anderson, 53, and said the teacher, Karen Elaine Smith, also 53, was his estranged wife.
Students at North Park Elementary School—which has around 500 students between kindergarten and sixth grade— were transported to a nearby campus, where they were “being well cared for, having snacks, playing games and watching a Disney movie,” the police department tweeted.
“Police operations are continuing to secure the area. However, we do believe the threat is down,” city Police Chief Jarrod Burguan said.
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San Bernardino, about an hour’s drive east of Los Angeles, became synonymous with gun violence when Syed Rizwan Farook and his wife Tashfeen Malik gunned down 14 people and wounded 22 others in December 2015 before being shot dead by police.
At the time, before the Pulse nightclub shooting in Orlando, Florida, it was the deadliest attack on US soil since September 11, 2001.
Pakistani-born Malik—who met her future US-born husband on a Muslim dating website and married him in Saudi Arabia—had pledged allegiance to the Islamic State group on Facebook and was instrumental in radicalising him.
A number of commemorative events, including a memorial mass, a vigil and a remembrance ceremony, were held in December to mark the one-year anniversary of the attack.
Monday's shooting will likely reignite the debate on gun violence in the US, where attempts to put in place tougher gun control measures have failed, despite a series of mass killings.
In one of the most notorious school shootings in modern US history, 20 children and six staff were massacred in 2012 at Sandy Hook Elementary in Newtown, Connecticut.
In June last year, 49 people were killed in a shooting rampage at Pulse, a gay nightclub in Orlando, Florida.
More than 10 out of every 100,000 Americans die every year from guns, including suicides, a rate far higher than in other Western countries.