Alleged Charlie Hebdo Shooters Were on US No-Fly List Days after magazine massacre in Paris, details emerge in lives of suspects.

 -- The two men who allegedly killed 12 people in a daytime massacre in Paris were listed in an American terrorist database and were on the U.S. no-fly list for years, a senior U.S. intelligence official said today.

The men, identified by French authorities as brothers Said and Cherif Kouachi, remain on the loose in France, despite a massive mobilization of French police forces since they allegedly attacked the Paris-based satirical magazine Charlie Hebdo Wednesday.

The senior U.S. official declined to say why the brothers’ information was on the American security lists, but information emerged today that the younger brother, Cherif, had been arrested and convicted in the mid-2000s on terror-related charges. Cherif, now 32, was briefly featured in a 2005 French television documentary as an aspiring rap musician-turned-jihadist.

At the time, Cherif had told a French court that the scandal at Abu Ghraib prison and the influence of a young religious leader convinced him to give up rapping to prepare himself for jihad abroad, according to local reports.

The French outlet Le Monde reported that Cherif was again imprisoned in 2010 and accused of terror-related crimes, but was released due to insufficient evidence.

Now, French officials say that he and his older brother, 34-year-old Said, both born in Paris, should be considered armed and dangerous.

Counter-terrorism experts told ABC News the Kouachi brothers appeared to be well trained, and videos of the attack display a menacing level of skill with weapons and combat tactics as they calmly execute a police officer at point-blank range. Their efficiency in the videos of the attack is evidence of careful preparation, according to Richard Clarke, former White House counter-terrorism advisor and current ABC News consultant.

“This does not look at all like a spontaneous attack or a lone wolf attack,” Clarke told ABC News. “This looks like a team that was selected, trained, probably over the course of a long period of time, and sent in with this particular target in mind.”

Charlie Hebdo, the satirical magazine that came under attack, and Stephane Charbonnier, its editor, had long been the subject of threats from Islamic extremist groups. In 2013, al-Qaeda published a hit list with Charbonnier’s name and photo. Before that, in 2011, the magazine’s previous office was firebombed in what is seen as a revenge attack for publishing crude cartoons of the Prophet Muhammad. The terror group ISIS had also threatened the magazine.

But despite publishing cartoons that offended practitioners of many religions, Charbonnier stood by the magazine and its artists’ rights to create controversial images.

“Without freedom of speech, we are dead,” Charbonnier told ABC News after the November 2011 firebombing of the Charlie Hebdo office.

Charbonnier was among the 12 killed during Wednesday’s attack, which took place during the magazine’s editorial meeting.

The database containing the brothers’ information, the Terrorist Identities Datamart Environment (TIDE), is the U.S. government’s central repository for information on individuals known or suspected to have been involved in, or aided, terrorism. As of 2013, the database held more than 1 million names, according to the National Counterterrorism Center.