The two French brothers wanted in the Paris magazine-office bloodbath were already known to French and US counter-terrorism authorities, and had been put on the American no-fly list.
A US official said the older brother, Said Kouachi, had travelled to Yemen. It was unclear whether he was there to join up with extremist groups like Al-Qaeda.
Witnesses said the gunmen who carried out Wednesday's shooting rampage claimed allegiance to al-Qaeda's offshoot in Yemen.
The details emerged as French authorities conducted an all-out man-hunt for the Kouachi brothers - Cherif, 32, and Said, 34 - in the attack that killed 12 people at the offices of Charlie Hebdo, a weekly that lampooned radical Islam and the Prophet Mohammed.
They were identified as suspects after the older brother's ID card was found in the getaway car, authorities said.
Both men - the Paris-born offspring of Algerian parents - were orphaned at a young age.
Cherif, a former pizza delivery man, had appeared in a 2005 French TV documentary on Islamic extremism and was sentenced to 18 months in prison in 2008 for trying to join fighters battling in Iraq.
US officials would not say if the Kouachis were believed to have fought in the Middle East with extremist groups.
It would not be unusual for known extremists to be placed on the US no-fly list.
A French security official, speaking on condition of anonymity because of the sensitivity of the matter, said that American authorities had shared intelligence with France indicating that Said had travelled to Yemen several years ago for training, and were seeking to verify its accuracy.
Before the words of a radical preacher persuaded him to book a flight to Syria to wage holy war, Cherif Kouachi was a ladies' man who belted out rap lyrics.
It was the teachings of a firebrand Muslim preacher that put him on the path to jihad in his rough-and-tumble neighbourhood of northeastern Paris, Kouachi was quoted as saying in the documentary.
Associated Press reporters who covered his 2008 trial, which exposed a recruiting pipeline for Muslim holy war in the multi-ethnic and working-class 19th arrondissement of Paris, recalled a skinny young defendant who appeared very nervous in court.
Cherif Kouachi's lawyer said at the time that his client had fallen in with the wrong crowd.
During the trial, Kouachi was said to have undergone only minimal training for combat - going jogging in a Paris park to shape up and learning how a Kalashnikov automatic rifle works by studying a sketch.
He was described at the time as a reluctant holy warrior, relieved to have been stopped by French counter-espionage officials from taking a Syria-bound flight that was ultimately supposed to lead him to the battlefields of Iraq.
French Interior Minister Bernard Cazeneuve, however, said Thursday that Kouachi had been described by fellow would-be jihadis at the time as "violently anti-Semitic".
Imprisonment changed him, his former lawyer Vincent Ollivier told Le Parisien newspaper in a story published Thursday.
Kouachi became closed off and unresponsive and started growing a beard, the lawyer said, adding that he wondered whether the stint behind bars transformed his client into a ticking time bomb.
After he was released from prison, he worked in a supermarket's fish section in the Paris suburbs for six months beginning in 2009. Supervisors said he gave no cause for concern.
In 2010, police detained him again in a probe of an alleged plot to free an Islamic militant sentenced to life in prison for bombing a Paris train line in 1995. Kouachi was ultimately released with no charges ever brought.
Much less has become public about Said, but Cazeneuve said the jobless resident of the city of Reims was also known to authorities, despite having never been prosecuted, because he was "on the periphery" of the illegal activities his younger sibling was involved in.
In Reims, about 145km northeast of Paris, Said frequented a prayer room on the ground floor of an apartment building, according to the local imam, Abdul-Hamid al-Khalifa.
Al-Khalifa told the AP that Said wore traditional North Africa clothes to prayers and didn't mix much - if at all - with other worshippers.
"Typically, he'd come late to prayers and leave right when they were done," Al-Khalifa said in a telephone interview.
A third suspect identified by French authorities in the attack turned himself in Wednesday night.
Mourad Hamyd, 18, reported by the French news agency AFP to be Cherif's brother-in-law, surrendered at a police station after learning his name had been linked to the case in the news, said Agnes Thibault-Lecuivre, spokeswoman for the Paris prosecutor.
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