Anwar al-Awlaki at Dar al-Hijrah mosque in 2001 in Falls Church, Va. (Tracy Woodward/WASHINGTON POST)

The attack in San Bernardino, Calif., has been widely interpreted as an indication of the expanding reach of the Islamic State. The shooters appeared to declare their loyalty to the terrorist group after they launched their assault, and President Obama has responded with vows to intensify the U.S. military campaign against the Islamic State in Iraq and Syria.

But new details released Thursday suggest that the San Bernardino plot was more deeply rooted in the ideology of al-Qaeda and the influence of its U.S.-born cleric Anwar al-Awlaki — a link that serves as the latest reminder of the latent threat posed by Awlaki’s followers four years after he was killed by a CIA drone.

Awlaki’s name appears repeatedly in a federal indictment that provides the most detailed chronology to date of the San Bernardino plot. Awlaki’s online teachings appear to trace the path to radicalization for one of the shooters, Syed Rizwan Farook, as well as a former neighbor, Enrique Marquez Jr., who bought rifles used to kill 14 people in San Bernardino this month.

The two men immersed themselves in Awlaki’s teachings for years without attracting attention from the authorities. They subscribed to Awlaki’s lecture series, including one titled “The Hereafter.” They clicked through online issues of Inspire, the ­English-language al-Qaeda magazine that often served as Awlaki’s platform.

As they progressed from eager pupils to willing attackers, the two studied other chapters of the al-Qaeda playbook, assembling pipe bombs with Christmas-light filaments and other components based on a recipe from the summer 2010 issue of Inspire.

After the mass shooting in San Bernardino, Calif., on Dec. 2, that left 14 people dead, the FBI is unearthing more information about who was involved in the attack. Here's what we know about Syed Rizwan Farook, his wife Tashfeen Malik and his neighbor Enrique Marquez. (The Washington Post)

They planned to attack a community college and a California freeway but abandoned the ideas for different reasons, according to the criminal complaint, including the arrests of three men in the area for plotting to kill Americans in Afghanistan.

“The conventional wisdom that the carnage in California was inspired by the Islamic State is wrong,” said Bruce Riedel, a former CIA terrorism analyst now at the Brookings Institution. The attackers were “not radicalized watching videos or social media in 2015,” Riedel said, but “marinated in extremist ideology and planning for a long period of time before they finally made the decision to act.”

The presumed link between the San Bernardino shooting and the Islamic State is based in part on a garbled posting on a Facebook page associated with the second shooter, Tashfeen Malik, as the assault got underway.

“We pledge allegiance to Khalifa bu bkr al bhaghdadi al quraishi,” the posting said, an apparent reference to Abu Bakhr ­al-Baghdadi, the Islamic State leader.

U.S. intelligence officials and experts said that Malik’s declaration appears to reflect an effort by the attackers to associate themselves with the most ascendent brand among extremist groups rather than carefully considered ideological preference.

But the criminal complaint reveals a more complicated set of motives at least for Farook, Malik’s husband, who seemed to embrace al-Qaeda in the Arabian Peninsula, the Yemen-based affiliate that sheltered Awlaki after he fled the United States.

The San Bernardino indictment depicts Farook as particularly enamored of Awlaki, to the extent that he began discussing his interest in traveling to Yemen to join AQAP, as the group is known, in 2011.

The San Bernardino killers' network of interest.

AQAP was behind an attempt to blow up a Detroit-bound airliner on Christmas Day in 2009, as well as a scheme the following year to smuggle explosives on U.S.-bound cargo planes.

A footnote in the indictment provides an unnerving summary of Awlaki’s legacy, stating that the 2005 London subway bombings, a 2006 plan to detonate truck bombs in Canada and the 2010 Times Square attempted attack all involved militants who “had listened to or viewed Awlaki lectures prior to their participation.”

Awlaki was killed in September 2011. The CIA killed the leader of AQAP in a strike four months ago, but U.S. counterterrorism operations have been badly damaged by the overthrow of the Yemeni government in 2014.

Despite the links to Awlaki, the U.S. response to San Bernardino has focused mainly on the Islamic State. In President Obama’s address four days after the shootings, he pledged that the United States would redouble its efforts to “stop ISIL’s operations — to disrupt plots, cut off their financing and prevent them from recruiting more fighters.”

This was one of 19 references Obama made to the Islamic State, also known as ISIL, in the speech. Al-Qaeda was mentioned three times but not its affiliate in Yemen.

The Islamic State split from al-Qaeda last year and has rapidly eclipsed its former parent by seizing territory, launching high­-profile attacks that include the recent assaults in Paris, and assembling a modern propaganda machine to draw thousands of recruits.

The Islamic State has posted hundreds of professionally produced videos online, including dozens that depict gruesome executions. The organization’s large presence on Twitter and other social-media platforms has made al-Qaeda — especially the core leadership headed by Aymen al-Zawahiri — appear increasingly antiquated.

But experts said that Awlaki’s work still resonates online because he delivered a compelling Islamist narrative in an approachable American voice.

“Awlaki has a timeless and even universal message of radicalization and resistance that is completely separate to whatever organization he hitched his fortunes to until the time of his death,” said Bruce Hoffman, a terrorism expert at Georgetown University. For those exploring online radicalism, Hoffman said, “Awlaki is a much deeper well to draw from than [Islamic State] tweets.”

There is no indication in the indictment that Farook or Marquez were still enamored of Awlaki in recent years. Much of their exposure to his extensive online library came during 2010 and 2011, according to the document.

The Islamic State and al-Qaeda are increasingly intense rivals in Syria, Yemen, Afghanistan and other regions where they are competing for territory and influence. But experts said the San Bernardino case and others indicate that the groups’ followers beyond those battle zones feel less compelled to choose sides.

The November attacks that killed 130 people in Paris were carried out by militants who had traveled to Syria and were closely aligned with the Islamic State. But the assault in the French capital earlier this year, which targeted the offices of the satirical newspaper Charlie Hebdo, involved gunmen who seemed to straddle the competing terrorist groups.

One of the gunmen in that last incident had received training from al-Qaeda in Yemen and apparently met Awlaki. But the shooter, who was killed after taking hostages in a kosher market, had declared his allegiance to the Islamic State.

“I think you can be a supporter of Awlaki and Baghdadi at the same time,” Riedel said. “The difference matters a lot if you are in Syria. I don’t think it matters that much when you are in San Bernardino or Paris.”