WASHINGTON (Reuters) - President Barack Obama will tell Americans on Sunday what the government is doing to keep them safe following the mass killing by a couple in California last week that is being investigated as an act of terrorism, but no major shift in strategy was expected.

Mourners gather around a makeshift memorial in honor of victims following Wednesday's attack in San Bernardino, California December 5, 2015. REUTERS/Sandy Huffaker

In a rare Oval Office address at 8 p.m. EST (0100 GMT), Obama will lay out the security steps the United States has taken since Islamic State’s Nov. 13 assaults in Paris that killed 130 people, as well as since the Sept. 11, 2001, attacks on the United States, an administration official told Reuters.

Obama said during a recent visit by French President Francois Hollande to the White House after the Paris attacks that the United States and its coalition would step up efforts to counter Islamic State. He is likely to repeat that message from the Oval Office and, despite Republican criticism that he has not done enough to fight Islamic State, is not expected to unveil a major change in strategy.

Obama also will say he will use “every single aspect of American power” to destroy the militant group, a U.S. official said.

The official said Obama also will give an update on the investigation of U.S.-born Syed Rizwan Farook, 28, and his Pakistani wife, Tashfeen Malik, 29, in the San Bernardino, California, shooting and express confidence that the United States will prevail over terrorist threats.

Another senior U.S. government source told Reuters that investigators are trying to determine if Malik, who Farook brought to the United States on a fiancee visa in 2014, had radicalized her husband’s Muslim faith and what sort of contacts she had with Islamic militants in Saudi Arabia and Pakistan, where she went to pharmacy school.

The couple had four guns, some 6,000 rounds of ammunition and a dozen pipe bombs either with them or at their rented townhouse in Redlands. The source said investigators are becoming increasingly convinced they were planning multiple attacks but are unsure of the targets.

U.S. Homeland Security Secretary Jeh Johnson told the New York Times the Obama administration was rethinking its approach to domestic terror because of the San Bernardino assault. He said the United States should beef up airline security by increasing the number of agents in overseas airports, bolstering standards for visa-waiver programs and improving communications between officials and Muslim communities to help locate threats.

U.S. Attorney General Loretta Lynch, appearing on NBC’s “Meet the Press” show, said Obama may call for congressional action.

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The San Bernardino attack could give Obama momentum with Congress, which has not granted his February request for formal authorization of the use of military force against Islamic State. Republicans say it is too limited in scope to defeat Islamic State while some liberal Democrats have opposed it as too open-ended.

White House officials say Obama has the power as commander in chief to press the fight against Islamic State even without the formal congressional authorization but wants buy-in from lawmakers to demonstrate a unified front.

STILL SEEKING MOTIVE

Lynch said the Federal Bureau of Investigation is focusing on what led Farook and Malik to open fire at a holiday party with Farook’s co-workers in San Bernardino on Wednesday.

Fourteen people died and 21 were wounded before police killed Farook and Malik in a street shootout a few hours later. Federal officials said the attack was an act of terrorism and are trying to determine any links to Islamic State.

Lynch said investigators had conducted 300 interviews and searched several sites related to last week’s attack. Neither Farook nor Malik was on the list of 900 U.S. residents under FBI investigation for suspected ties to Islamic extremist groups.

“I can’t say definitively right now what led either of these two people to pick up guns and become murderers,” Lynch said. “We’re looking at everything we can find out about these two killers’ lives - how they grew up, where they grew up, how they met.

“These two individuals - self-radicalized, radicalized by others - that’s what we definitely want to learn so we can figure out how to best prevent this again.”

Facebook confirmed that comments praising Islamic State were posted about the time of the shooting to an account set up by Malik under an alias. Islamic State said on Saturday the couple were among its followers.

Even before San Bernardino, concerns about the United States as a terror target were elevated by the Islamic State attacks in Paris and plans to admit 10,000 refugees from Syria’s civil war.

Lynch said Obama’s address to the nation would focus on what the federal government has done to ensure the safety of Americans and call on them “”not give into fear at this time.”

The San Bernardino attacks will add to the debate on how much power law enforcement and intelligence agencies need to ensure safety from extremists. Complaints about the intrusiveness of the National Security Administration led a week ago to the shutting down of its daily vacuuming of millions of Americans’ phone records.

Democratic presidential candidate Hillary Clinton, who served as Obama’s secretary of state, said she expected the president to announce a new approach to Islamic State that would be “an intensification of the existing strategy,” which focuses on coalition air strikes.

Republican presidential candidates demanded tighter security standards to fight terror with Senator Lindsey Graham of South Carolina, who is near the bottom of polls in the field, said Islamic State was planning a “9-11 style attack” on the United States.

“It is a matter of time (until) we get hit by ISIL. Not some lone wolf attack but hardened terrorists are coming here to hit us hard if we don’t hit them first,” he said on “Meet the Press.”

Republican presidential candidate Ted Cruz said if he is elected in November 2016, the U.S. approach to Islamic State should be to “carpet bomb them into oblivion.”