U.S. President Barack Obama is making a rare primetime television address to the country late Sunday, hoping to calm Americans' fears about terrorist attacks and lay out the United States campaign against "the broader threat of terrorism."
He also plans to update the country on the investigation into last week's mass shooting in California, where U.S.-born Syed Rizwan Farook and his Pakistani wife Tashfeen Malik killed 14 people and wounded another 21 in a hail of gunfire at a holiday gathering of local government workers in San Bernardino.
The country's top law enforcement official, Attorney General Loretta Lynch, told NBC's Meet the Press that Obama would call on the American people "to pull out the best in themselves and not give in to fear."
In advance of the speech, the White House said Obama will discuss the nature of the terrorist threat, "how it has evolved, and how we will defeat it."
Obama "will reiterate his firm conviction" that Islamic State insurgents "will be destroyed and that the United States must draw upon our values, our unwavering commitment to justice, equality and freedom, to prevail over terrorist groups that use violence to advance a destructive ideology."
Obama will speak from the Oval Office in the White House, where U.S. presidents have delivered some of their most significant speeches. Obama last gave a televised speech from the Oval Office to announce the end of U.S. ground combat operations in Iraq in 2010.
The Federal Bureau of Investigation is continuing to investigate what led Farook and Malik to carry out the California attack.
Lynch said investigators are "trying to learn everything" about the couple, "what led these two people to become murderers." She said authorities are tracking Malik's Pakistani background, her relationship with Farook and how they came to attack the site they did, a government center that offered services for developmentally disabled people.
But she said at this point, investigators do not believe the couple was part of a larger terrorist network.
Authorities on Saturday raided a house in the city of Redlands they believe was home to a man who bought two assault rifles used in the shooting. Farook used to live next door to the house and neighbors described him as a friend of the man who lived there, but the FBI has declined to give details of the raid. The man has not been charged with any crimes and checked himself into a mental health facility after Wednesday's attack.
WATCH: Related video on search of couple's apartment
Farook and Malik fled after the shootings, but their car was later spotted and they were killed as they exchanged gunfire with police. Police found 12 pipe bombs, bomb-making materials and thousands of rounds of ammunition in their home.
The FBI is investigating the attack as an act of terrorism. White House officials said there is no indication the couple was part of an organized group or terror cell.
The Islamic State group claimed on its English-language radio broadcast Saturday the two were its "soldiers," and in an Arabic broadcast called them "supporters." The militant group did not explicitly claim responsibility for the attack.
Malik posted a message on Facebook around the time of the attack pledging allegiance to Islamic State.
Pledges of loyalty to IS leader Abu Bakr al-Baghdadi have been reportedly found in social media posts by others who carried out mass killings. Some Islamic State supporters posted messages on Arabic social media hailing Wednesday's shootings and congratulating the killers. Some promised more attacks in the the United States.
The FBI said a number of pieces of evidence had turned up indicating that this was a terrorist act, including signs the massacre was extensively planned. Investigators are also looking at two crushed cell phones the couple apparently threw away near the site of the killings.
Two attorneys representing the Farook family criticized the media for what they say is a rush to judgment that this was a terrorist attack simply because the suspects were Muslim.
Lawyers David Chesley and Mohammad Abuershaid said they and their clients met with the FBI for four hours, and the family is totally shocked because, they said, the couple never showed extremist or aggressive behavior and never mentioned Islamic State.
Chesley said the FBI has no "clear smoking gun" evidence pointing to terrorism.
Friends, family, and co-workers said they have no clues what sparked the couple to carry out their massacre. Neither had a criminal record and were not on any government watch list.
The Chicago-born Farook was a local government health inspector in San Bernardino. Friends said he was dedicated to Islam, cordial, liberal-minded and well-liked.
He met the Pakistan-born Malik through an online dating site. The two met in Saudi Arabia and married. She came to the United States on a fiancee visa. The couple were parents of a six-month-old girl, whom they dropped off at a relative's house before launching the attack. Some of Farook's friends said he returned from Saudi Arabia a changed man.
The weapons used in San Bernardino were bought legally. President Obama said it is "just too easy" for people to buy guns in the United States. He said what happened Wednesday should spur lawmakers in Washington "to take basic steps to make it harder ... to get weapons."
Senate Republicans Thursday rejected an amendment that would have worked toward blocking suspected or known terrorists from being able to legally buy guns.