Moments after armed assailants stormed the Inland Regional Center, a San Bernardino police officer broadcast an urgent message.
"We have two witnesses who watched the whole thing start," the officer said. "They said there are three shooters with rifles without a doubt. They said definitely three."
Hours later, police Chief Jarrod Burguan repeated the number during a news conference that was broadcast nationwide, telling reporters that there may have been as many as three shooters involved in the Dec. 2 attack.
In the months since, FBI and police say they have found no evidence that the attack was carried out by anyone other than Syed Rizwan Farook and his wife, Tashfeen Malik.
But the early reports of a possible third shooter — a source of worry for first-responders racing to the scene — continue to bedevil authorities.
A handful of victims interviewed by police still maintain they saw a third attacker. The father of one victim raised the possibility this week while weighing in on the legal dispute over investigators' attempts to gain access to Farook's work iPhone. And Burguan said residents still ask him about an additional assailant when he attends public forums.
"The questions about this third shooter have persisted, and I understand why," Burguan said. "Our dispatch radios have been recorded, they have been played live. We know there was information that went out over the air. That was all fog of war."
The lingering concerns about a third shooter underscore a key challenge police face when trying to make sense of conflicting witness accounts in rapidly unfolding, violent crimes.
Studies have found that witnesses who experience a highly stressful or traumatic event are less likely to accurately recall details. An exhausting news cycle that helps spread early information that is often wrong also serves to feed false narratives even after a better picture of what happened emerges, according to police and forensic psychologists.
"Once the misinformation is embedded, it becomes a part of their memory," said Deryn Strange, a professor of psychology at John Jay School of Criminal Justice in New York. "It is exceptionally difficult to get rid of that information because we don't really take on corrections particularly easily."
Federal investigators have said that none of the evidence they have so far collected — including weapons used in the attack — indicate the involvement of a third shooter.
Burguan said ballistics evidence shows only two weapons were used in the Inland Regional Center. Police have interviewed several hundred people in connection with the investigation, and the vast majority reported that they saw two shooters, he said. There were no surveillance cameras in the room where the shooting occurred.
"In the absence of video, you rely on forensics and witness accounts," said Laura Eimiller, an FBI spokeswoman.
As the probe continues, FBI officials said they won't rule out the possibility of a third shooter — or any other potential scenario. Last month, the federal government called on Apple Inc. to help FBI agents unlock an iPhone used by Farook, as part of the bureau's effort to figure out if he communicated with anyone else about the terror plot.
The ongoing investigation into the shooting can help fuel public concerns that another person might have helped in the attack, said Strange, whose research focuses on false memories held by witnesses to traumatic events.
Relatives of those killed in the terrorist attack and law enforcement leaders also threw their weight behind the FBI in court filings this week, arguing that information on Farook's phone could lead to information about possible co-conspirators or a third shooter.
Mark Sandefur, whose son was killed in the massacre, said in a letter to Apple's chief executive that several survivors recounted "bone-chilling stories" involving a third attacker.
"Recovery of information from the iPhone in question may not lead to anything new," Sandefur wrote in the letter. "But, what if there is evidence pointing to a third shooter?"
San Bernardino County Dist. Atty. Michael Ramos filed a brief backing the FBI that cited two 911 calls that reported a third shooter.
"Although the reports of three individuals were not corroborated, and may ultimately be incorrect, the fact remains that the information contained solely on the seized iPhone could provide evidence to identify as of yet unknown co-conspirators," Ramos wrote.
Other major disasters or violent crimes have produced witness accounts that were later discounted by authorities but nonetheless helped spur enduring concerns or alternative theories about what occurred.
During the 1997 North Hollywood bank robbery that left two gunmen dead and several police officers and witnesses injured, early reports surfaced of a third heavily-armed suspect. Police responded by knocking down doors, even a shed, to search the residential area around the bank, and an ambulance racing to the bloody scene was also delayed out of fear that a third assailant was nearby.
Over the next few days, police said the reports had been wrong. Even so, years later, a woman who lived nearby came forward to insist she had seen a heavily armed man run from the area of the shootout into a shed near her apartment.
Nearly two decades after TWA Flight 800 crashed into the Atlantic Ocean off the coast of New York in 1996, some witnesses remain insistent that a U.S. Navy missile brought down the airliner. The National Transportation Safety Board spent four years investigating the crash, and in 2000 ruled that a fuel tank explosion probably caused the jet to break apart, killing all 230 people on board.
In 2013, a group that included investigators from the original NTSB probe urged the agency to reconsider its earlier findings, arguing that radar and other evidence supported witness accounts that "a detonation or high-velocity explosion" caused the crash.
The NTSB denied the request, saying that the group's analysis was flawed. "None of the physical evidence supports the theory that the streak of light observed by some witnesses was a missile," the agency declared.
During the 2013 manhunt for Christopher Dorner, an ex-LAPD officer who killed four people and wounded three others, the military veteran's ability to elude authorities for days sparked concerns among investigators that he was being helped by others, said San Bernardino County Sheriff John McMahon.
Dorner was eventually cornered and killed himself in a gun-battle with law enforcement officers in Big Bear. Investigators concluded he had been acting alone, but they did not publicly talk about their earlier search for accomplices, McMahon said. That decision, he said, helped end any public discussion or false narrative about Dorner's rampage.
"It is absolutely human nature for folks to think there's got to be more to this," McMahon said.
In the December attack on the Inland Regional Center, the early reports of a third shooter weighed heavily on the minds of those rushing to the aid of victims.
Michael Neeki, a trauma physician who entered the building with a county SWAT team, was already struggling to tune out panicked sobs and shrieks echoing throughout the facility as he tried to evacuate victims. With each step, he said, he wondered whether another round of gunfire or an explosion could happen.
Alex and Nina Jabourian, a husband-and-wife who work as emergency medicine residents at Arrowhead Regional Medical Center in nearby Colton, shared Neeki's concern as they arrived at the scene of the firefight that left Farook and Malik dead hours later.
They had been told during a debriefing that there may have been three shooters. Even after Farook and Malik had been killed, reports of another armed assailant were coming over police radios.
"Right now we have one down outside the car, one down inside the car. From what we understand, one is on the run," a dispatcher said, according to recordings of transmissions that followed the gun battle.
"It was worrisome because we didn't have complete information about what was going on," Nina Jabourian recalled. "We didn't know if it was safe to be there."
Months later, Jabourian says that she and her husband no longer wonder whether there was a third shooter. She attributes those concerns to the chaos of the day.
The dispatch of a possible third assailant during the gun battle came after several witnesses reported seeing someone run from the shootout and hop over several fences, Burguan, the police chief, said. He said officers caught the fleeing man and determined he was simply running from the gunfire.
"We deal with this all the time with eyewitnesses," Burguan said. "Four different people can see the same exact incident and have four different stories to what they saw."
Times staff writers Joseph Serna and Richard Winton contributed to this report.