Federal investigators in California on Tuesday appealed for the public’s help in piecing together the whereabouts of the two San Bernardino shooters during a brief period after the massacre, AFP reports.

David Bowdich, head of the FBI office in Los Angeles, said that while investigators had largely determined the movements of Syed Farook and his wife Tashfeen Malik on the day of the massacre December 2, there were 18 minutes unaccounted for after the shooting that left 14 people dead and 22 injured.

He said investigators have been unable to determine where the couple was between 12:59 p.m. and 1:17 p.m., according to AFP.

The period falls between the time of the shooting, which took place at 10:59 a.m., and the time the couple was spotted by police and died in a firefight at around 4:00 p.m.

“So why is that 18 minutes so important?” Bowdich told a press conference. “It’s important because we want to ensure that we know whether or not they stopped at any locations, any residences, any businesses that we don’t already know about.”

He noted it was possible that during those 18 minutes the couple may have stopped at a storage facility or had contact with someone.

“We just don’t know,” Bowdich was quoted as having said. “It may not be an important fact, but until we close that gap we don’t know.”

The FBI has said that Farook and Malik were both radicalized “for quite some time” and had taken target practice at Los Angeles-area shooting ranges, and once “within days” of the massacre.

It is believed Farook had contact with people from at least two terrorist organizations overseas, and investigators have also said Malik had pledged allegiance to the Islamic State (ISIS) group in a Facebook posting.

Bowdich on Tuesday said investigators had determined that after the attack the couple drove around the San Bernardino area, where they lived, stopping at several locations, including a lake.

He said a search of the lake had produced nothing linked to the investigation.

Meanwhile, last week a former neighbor of Farook who is believed to have supplied the shooters with their weapons was indicted on charges of conspiring to provide material support to terrorists.