Islamic State group supporters celebrated the fatal shooting of 14 people in San Bernardino, California, Wednesday by tweeting "America Burning." The group, also known as ISIS or ISIL, has not claimed responsibility for the shootings, but the radical group based in Syria and Iraq has threatened to carry out attacks across the U.S.

“Three lions made us proud. They are still alive,” one ISIS adherent tweeted in Arabic after the shootings at Inland Regional Center in San Bernardino, Vocativ reported exclusively. “California streets are full with soldiers with heavy weapons. The United States is burning #America_Burning #Takbir”

Another ISIS supporter posted in reference to the shooting on Twitter, “God is the greatest. May god spread fear in the homes of the Crusaders.” Another ISIS extremist wrote: “Let America know a new era #California #America_burning.”

President Barack Obama said Thursday from the Oval Office that the motive behind the killings was unknown but terrorism could be involved. The shooting in California carried out by a husband and wife also left at least 21 wounded in the deadliest mass shooting in the U.S. since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in 2012.

A police search for the suspects resulted in a deadly shootout, with both suspects killed. A third person was detained, but police later said only the two suspects were involved in the attack. The man involved in the shootings was Syed Farook, a staff employee at the San Bernardino County Public Health Department.

Farook’s father said his son was a “very religious” Muslim. “He would go to work, come back, go to pray, come back,” his father told the New York Daily News.

Farook’s brother-in-law, Farhan Khan, told reporters during a news conference in Anaheim, California, organized by the Council on American-Islamic Relations (CAIR) that he was shocked. “I cannot express how sad I am for what happened today,” he said. “My condolences to the people who lost their lives. I am in shock that something like this could happen.”

Twitter users were quick to blame the shooting on ISIS. Google Trends found that the terms “ISIS” and “California” were heavily searched together Wednesday.

Earlier this week, a California man pleaded guilty to planning to join the group in Syria. Nicholas Michael Teausant, 22, was charged with attempting to provide material support to a foreign terrorist organization, the Jerusalem Post reported.