Cherif Kouachi, one of the suspected gunmen in the attack on Charlie Hebdo newspaper in Paris, was seen in a 2005 episode of “Pièces à Conviction” on young jihadists. Kouachi was killed Jan. 9, 2015, during a shootout with police. (National Audiovisual Institute)

French security services are likely to face intense pressure to explain how known militants — including one trained by an al-Qaeda affiliate in Yemen — faced no apparent scrutiny before they launched this week’s terrorist attacks in Paris, including the daytime assault on a a satirical newspaper, a long-declared Islamist target.

The search for answers is likely to focus on a three-year period preceding this week’s shooting during which two of the alleged gunmen, Said and Chérif Kouachi, seemingly dropped out of the view of French intelligence services as well as their U.S. counterparts.

U.S. counterterrorism officials said they have spent the days since the attack on the newspaper Charlie Hebdo scouring databases maintained by the CIA and National Security Agency, among others, for clues to whether the Kouachis kept in communication with al-Qaeda in the Arabian Peninsula (AQAP) during what one official described as a “dark period” in a decade-long chronology.

Chérif Kouachi, speaking to a French journalist shortly before he was killed by security forces Friday, said he and his brother were acting on behalf of “al-Qaeda in Yemen,” and that the U.S.-born cleric and operative Anwar al-Awlaki had played a role in the training received by Said Kouachi during his trip to Yemen in 2011.

A member of AQAP said in a statement that the group directed the Paris attack, but U.S. intelligence officials said they had found no evidence to support the claim. They said the Yemeni group, which has been overshadowed in the past year by the Islamic State in Syria and Iraq, may simply have seized on the opportunity to associate itself with the attack.

Two hostage situations related to Wednesday's massacre at the satirical newspaper Charlie Hebdo ended Friday evening Paris time when police simultaneously stormed both sites. All three suspects were killed.

But security officials said the Paris attack has raised anxiety beyond concern over a re-emergence of AQAP.

A senior U.S. official noted that more than 3,000 European citizens, including at least 1,000 from France, have flocked to Syria to fight with the Islamic State and other militant groups. Some have returned to Europe and could perhaps go for years without drawing attention — much as the Kouachis did — before it is clear whether they pose a threat.

“They are potential time bombs,” the official said.

If so, the fuse leading up to the Paris attack often seemed to burn in plain view.

The target, Charlie Hebdo, had long been in the sights of Islamist groups angered by the publication’s mocking of Islam. Its offices had been firebombed in 2011, and it was listed among AQAP’s priority targets last year in the group’s “Inspire” magazine.

The Kouachis had been under the scrutiny of French authorities at least as early as 2005, when the younger brother, Chérif, was arrested as he attempted to leave for Syria as part of an alleged plan to join insurgents in Iraq. He also appeared in a French television documentary on jihadist networks.

During three years in detention, French officials said, Chérif Kouachi became acquainted with another radical, Djamel Beghal, accused of planning an attack on the U.S. Embassy in Paris. Kouachi was released in 2008.

Three years later, Said Kouachi traveled to Yemen, apparently to seek training from AQAP, a group that had already set in motion a series of plots, including the failed attempt to blow up a U.S.-bound airliner on Christmas Day, 2009, with a bomb sewn into an operative’s underwear.

The brothers had also surfaced on counterterrorism screens in the United States. Lauren C. Anderson, the FBI’s top agent in Paris when Chérif Kouachi was arrested in early 2005, said he and others were placed on U.S. counterterrorism databases shortly afterwards.

“There were indications they were trying to go to Syria to get into Iraq,” said Anderson, who later directed FBI counterterrorism operations in New York. “That’s how they got on the no-fly list.”

That list would also likely have prompted the NSA and other spy agencies to scour U.S. intelligence databases for intercepted communications or other intelligence fragments connecting Chérif Kouachi to terrorist groups.

U.S. officials said they are also seeking to determine whether the older Kouachi met with Awlaki in 2011. Officials said they see that as plausible because Awlaki was in charge of AQAP’s external operations and presumably would have been acutely interested in a recruit from Europe. But they have uncovered no evidence of such an encounter.

Awlaki was killed in a CIA drone strike shortly after Kouachi’s return to France, raising speculation that the cleric’s death may account for the brothers’ extended period of inactivity — that they shelved plans or cut off communications with AQAP as part of a broader effort to maintain a lower profile.

Either way, the brothers appear to have faced diminishing levels of scrutiny from French security services that over the past two years have become increasingly consumed with tracking an exodus of French citizens to Syria.

France is widely seen as the most aggressive country in Europe in its surveillance of Islamists, as well as its willingness to seize its own citizens’ passports and take other measures to prevent them from departing for Syria’s civil war.

But even before this week’s attack, France has been at the center of a series of apparent security lapses.

Last year, French citizen Mehdi Nemouche killed four people in a shooting at a Jewish museum in Brussels, even though French authorities knew he had gone to fight in Syria and had been told by German officials that he had returned to Europe.

In 2012, a French national, Mohammed Merah, killed three Jewish schoolchildren, a rabbi and three French soldiers in a series of shootings in southwestern France. He had been on a U.S. no-fly list since 2010 after he was detained in Afghanistan and sent back to France, where he had a criminal record and was known for his extremist views.

Merah was killed in a shootout with French commandos in Toulouse on March 22, 2012. At the time, the French interior minister, asked if the security services could have done more, said that “expressing opinions, showing Salafist opinions” — a reference to a fundamentalist strain of Islam — “is not enough to bring someone before justice.”

Julie Tate and Karen DeYoung contributed to this report.