An officer spoke into the public airwaves, “We’ve got shots fired out the back window."
As Twitter eavesdropped, the hunt for Syed Rizwan Farook, 28, and Tashfeen Malik, 27 — the married couple behind the mass murder — climaxed that evening in a police shootout.
The scanner chatter, captured in this Soundcloud, revealed the responding officers' tactics ("We have the suspect vehicle stopped. We're going to extract him. Wait for the BearCat"). Some users pointed out such information could help the criminals, if they or a remote third party happened to be tuned in.
This problem led the Washington, D.C. police department to start encrypting channels in 2011. Only people with passwords or the right hardware can tune in.
"Whereas listeners used to be tied to stationary scanners, new technology has allowed people — and especially criminals — to listen to police communications on a smartphone from anywhere," Police Chief Cathy Lanier said at the time. "When a potential criminal can evade capture and learn, 'There's an app for that,' it's time to change our practices."
Criminals used the scanners to stay ahead of police, Lanier argued at a city council hearing. The open channels, she said, helped criminals pull off a series of carjackings.
Which raises the question: Police in the United States aren't legally obligated to broadcast their conversations, so why do many departments still grant this all-access pass?
The answer, according to Daniel Isom, former chief of the St. Louis police: The benefit exceeds the trouble.
"People listen because they want to know what's going on in their community," he said. "Maybe they could have heard about the situation in San Bernardino and fled the area. Or maybe they lived nearby, looked out the window and saw the [license plate number] on that SUV."
Besides, non-officers rarely rush to still-volatile crime scenes — at least in St. Louis, Isom said. He appreciates eager reporters, too; they publish information that helps the public spot a perpetrator.
But scanners can also spread misinformation. Take the manhunt after the Boston marathon bombing: Listeners heard names of people who weren't actually suspects. Some of those people received death threats.
Departments in California, Florida and New York, for example, have also encrypted their conversations. Some offer special radios or log-ins for the media. Others have replaced scanners with email blasts to crime reporters (who complain they're updated too late).
John Banzhaf, a law professor at George Washington University and former engineer, said the rise of smartphones created the dilemma for police departments. “In the past, people had to go out and buy a separate piece of equipment,” he said. “This involved some expense, and a bit of expertise to operate.”
But efforts to safeguard sensitive information now, he said, could actually slow down emergency responders during terrorist attacks and natural disasters. “Any system to get everyone in a major metropolitan area into one encrypted communications system is bound to fail from time to time,” Banzhaf said. “Possibly when instant communication and close coordination is most vital.”
Police officials in Greenwich, Conn. said they considered encrypting conversations but ultimately decided to keep channels open. "Because we've always retained the ability to encrypt traffic on a case-by-case basis when we need to, in a community like Greenwich, I think the transparency we achieve by allowing people to listen to our radio communications certainly outweighs any security concern we have," Capt. Mark Kordick told USA Today.
That's good news for all those nosy people on Twitter. In a world of constant chaos, police scanners have become a social event: We connect through tragedy.
"There's this built-in tendency to engage in surveillance," said Frankie Bailey, a criminal justice professor at the University at Albany, SUNY. "We want to know what's going on, the details, the inside story."