Investigators have accounted for much of the timeline of how Syed Rizwan Farook and his wife, Tashfeen Malik, came to arrive at a holiday party at the Inland Regional Center in San Bernardino and opened fire before they were spotted hours later in Redlands, California, and killed in a shootout with authorities.

The FBI on Tuesday appealed for the public's help in closing an 18-minute gap in the timeline of the San Bernardino mass shooting that killed 14 people on Dec. 2.

But between 12:59 and 1:17 p.m., FBI Asst. Director David Bowdich said the timeline goes dark.

"We want to make sure we know if they stopped at any location, any residence, any businesses; we want to make sure we know if they had contact with anyone that we don't already know about between that time," Bowdich said.

For much of the timeline that is known, Bowdich characterized the shooters' movements as erratic and "back and forth." The pair stopped briefly at a parking lot and Seccombe Lake in San Bernardino.

While he declined to speculate if whatever happened during the timeline gap could still present a danger to the public, "until we close that gap, we just don’t know for sure."

Investigators are asking the public for photographic, video, or other information to call the FBI at (800) 225-5324 to help close that gap.

Bowdich added that a search of nearby lake at a park revealed no items pertinent to the investigation.

