Syed Farook and Tashfeen Malik killed 14 of Mr Farook's colleagues at a San Bernardino community centre last month.
But the FBI are unsure what the couple did in the minutes after the attack, before a police pursuit and shootout in which they were killed.
Officials said the Muslim couple were radicalised but appeared to act alone.
David Bowdich, the assistant director in charge of the FBI's Los Angeles office, said there was no evidence that the attack was planned or directed by anyone overseas.
But he said it was important that they find out what the couple were doing - and if they contacted anyone - between 12:59 pm and 1:17pm on 2 December.
They drove around a lot after the shooting, he said, and they made a few stops, including one at a nearby lake.
But a search of the lake turned up nothing of interest.
Mr Bowditch also said there was no evidence the couple had any other targets other than the Inland Regional Center, where they opened fire on a holiday lunch event.
The 29-year-old Malik pledged allegiance to the so-called Islamic State on the day of the shooting.
The killing has been described by authorities as the deadliest terror attack on US soil since 9/11.
Authorities investigating a deadly shooting in California have appealed for help in filling an 18-minute gap in the movements of the perpetrators.