The US government appears not to have picked up on extremist messages exchanged during the online courtship of the married couple accused in the San Bernardino shooting.
Syed Rizwan Farook and his wife, Tashfeen Malik, discussed martyrdom and jihad online as early as 2013, according to closed-door briefings to Congress provided by federal officials.
But the couple never surfaced on law enforcement's radar and Malik was able to enter the US on a fiancee visa last year despite having professed radical views online, raising concerns among politicians about whether any red flags were missed in the last two years.
Meanwhile, the investigation into the shootings that left 14 dead last week continued in San Bernardino, where an FBI dive team searched a small, urban lake about three miles north of the shooting site.
FBI director James Comey and other senior American officials briefed members of Congress on Capitol Hill about aspects of their continuing investigation into the terror attack.
One official said information that the FBI has been able to glean about the couple comes from an examination of their electronic devices, rather than intercepts.
"Everyone's asking the same questions about how it is that law enforcement didn't know, or intelligence officials didn't know - that they could have flown under the radar and nothing gave an indication that they were a threat," said representative Jim Langevin, a Rhode island Democrat and member of the House homeland security Ccommittee.
Representative Adam Schiff, the top Democrat on the House permanent select committee on intelligence, said it was understood that Malik was subjected to an in-person interview during the application process for a visa.
Republican representative Will Hurd of Texas, a member of the House homeland security committee, said there is currently no evidence Malik's radicalisation would have been readily apparent when she was evaluated for a fiancee visa.
"I don't think there was missed information," he said. "It appears that there was not any evidence that would have been discoverable during an interview for a visa."
He declined to discuss what specifically led investigators to conclude that the couple had radicalised independently as early as 2013, but suggested the information did not come from intercepts.
Mr Comey has said Farook had been in communication with individuals who were being scrutinised by the FBI in terrorism investigations, but the contact he had was not enough to bring him onto the law enforcement radar.
"It's safe to say that the information about what happened prior to their marriage and to the attacks in San Bernardino was acquired through forensic investigations of these individual lives," Mr Hurd said. "These people weren't on the radar."
New revelations show a much deeper connection between Farook and Enrique Marquez, his friend who bought the assault rifles used in the shooting, than previously was disclosed. Mr Marquez has not been charged with a crime.
More than three years ago, Mr Marquez purchased the high-powered weapons that Farook and his wife used to open fire on a holiday gathering of Farook's fellow health inspectors on December 2, killing 14 people. Farook and Malik were killed hours later in a shootout with police, leaving behind a six-month-old daughter.
Investigators are trying to determine if Farook's path toward extremism predated 2013 and whether it led to plans to launch an attack in 2012, according to sources.
Mr Marquez and Farook "were plotting an actual attack" that year, including buying weapons, but became apprehensive and shelved the plan because of law enforcement activity and arrests in the area, said Idaho senator Jim Risch, a Republican who sits on the Senate select committee on intelligence.
Mr Marquez, 24, spoke to federal authorities after they raided his mother's Riverside house over the weekend. He and Farook were friends for years and became family last year, with a sister-in-law in common.
The two men were listed as witnesses on the marriage licence when Farook's brother, Raheel, wed a Russian woman in 2011.
Three years later, Raheel Farook and his wife Tatiana were witnesses to Mr Marquez's marriage to her sister Mariya Chernykh, according to records.
The ceremony took place at the Islamic Society of Corona-Norco, according to the marriage licence, though the mosque's facility manager denied it occurred there.