The mass shooting in San Bernardino in 2015 became a topic of the second presidential debate Sunday when Donald Trump repeated the false claim that several witnesses saw explosives at the home of the attackers but neglected to alert law enforcement.
After a Muslim woman asked the Republican candidate how he would help millions of Muslims in the U.S. overcome the stereotype that they pose a threat, Trump referred to the Dec. 2 mass shooting and attempted bombing that left 14 people dead and seriously injured 22 others.
"In San Bernardino, many people saw the bombs all over the apartment of the two people that killed 14 and wounded many, many people," Trump said. "Muslims have to report the problems when they see them," he later added.
Trump and his supporters have repeated variations of the claim before, but there is no evidence that witnesses saw weapons in the Redlands townhouse of Syed Rizwan Farook and Tashfeen Malik.
The FBI has said it had not been watching Farook and Malik prior to the massacre and did not receive advance warning that they were dangerous.
Friends, family and co-workers have said publicly that they did not know Farook and his wife had been plotting the attack.
The notion that Muslims should police each other for suspicious behavior sparked a viral hashtag on social media, #MuslimsReportStuff, with debate viewers offering humorous takes on Trump's comments about Muslims.
The hashtag quickly became a vehicle to criticize Trump's political platform and comment on the broader view of Muslims in American society. "Hello, I'd like to report a dangerous racist misogynist demagogue on my TV… yes, I'll hold," Zainab Chaudary wrote.
"Where is the call to all white people to report white terrorists?" Amelia Noor-Oshiro wrote on Twitter, later adding, "White terrorists kill far more than any other group!"
After the San Bernardino attack, investigators found a cache of weapons in the assailants' townhouse, including thousands of rounds of ammunition and a dozen pipe bombs. The house also had a workshop used to make explosive devices.
The claims about missed opportunities to avert the attack appear to originate in a second-hand account from one of the couple's neighbors.
Aaron Elswick told KTLA-TV Channel 5 that a friend of his who lived near Farook and Malik had spotted suspicious activity at the home, including the delivery of numerous packages. The couple also seemed to work in their garage at strange hours, he told the news station.
The friend apparently did not report it out of fear of racial profiling, he said. The identity of the friend was not reported.
Trump laid blame on the Muslim American community for a failure to report Farook and Malik's terrorist ambitions, and cited recent attacks in Orlando, San Bernardino and Paris to support his assertion.
"Muslims have to report the problems when they see them, and you know, there's always a reason for everything," Trump said, later adding, "If they don't do that, it's a very difficult situation for our country."
New Jersey Gov. Chris Christie made a similar claim during a Republican primary debate in January, stating that neighbors were aware of Farook and Malik's violent aims.
"These folks had weapons, they knew that they were talking about trying to take our country and attack it," Christie said, according to the fact-checking website PolitiFact.
That debate's moderator, Megyn Kelly, replied that the neighbors were not aware that the couple would launch an attack.
"They knew they were talking about attacking people," Christie said.
As the investigation into the San Bernardino attack unfolded, investigators learned that Farook and his former neighbor and childhood friend, Enrique Marquez Jr., had plotted in 2011 and 2012 to carry out attacks at Riverside City College and on the 91 Freeway.
Marquez is accused of buying two rifles for Farook as they prepared to carry out those attacks, as well as smokeless powder that was used to make pipe bombs that were left by Farook in the San Bernardino conference room where the attack occurred.
Marquez was working at a Wal-Mart on the day of the attack. The next day, he called 911 and told an operator that Farook had used his gun.
"How do you know it's your gun?" the 911 operator asked. Marquez said, "They can trace all the guns back to me."
Prosecutors filed federal charges against Marquez in the unrealized terrorist plot. He is also charged with unlawfully handing over the weapons to Farook. He has pleaded not guilty, and is awaiting trial.
Assistant U.S. Atty. Christopher Grigg has said there was no evidence that Marquez participated in the attack at the Inland Regional Center.
10:20 p.m.: The story was updated with information about the #MuslimsReportStuff hashtag.