WASHINGTON — The younger of the two brothers who killed 12 people in Paris last week most likely used his older brother’s passport in 2011 to travel to Yemen, where he received training and $20,000 from Al Qaeda’s affiliate there, presumably to finance attacks when he returned home to France.

American counterterrorism officials said on Wednesday that they now believed that Chérif Kouachi, the younger brother, was the aggressor in the attacks — not Saïd Kouachi, the older brother, as they first thought — but that Saïd may also have traveled to Yemen, as American and French authorities have said.

The fuller portrait of the brothers has emerged as an international effort is focused on determining who may have been behind the attack on the French newspaper Charlie Hebdo, and what direct role, if any, Al Qaeda, its affiliates or their bitter rival, the Islamic State, had in planning and ordering the assault. In a video and written statement, the Qaeda branch in Yemen on Wednesday formally claimed responsibility for the deadly assault. It said the target had been chosen by the Qaeda leadership but did not specify which leaders.

If the claim of direct responsibility holds up, it would make the attacks in France the deadliest planned and financed by Al Qaeda on Western soil since the transit bombings in London in 2005 that killed 52 people. And it would serve as a reminder of the continued danger from the group at a time when much of the attention of Europe and the United States has shifted to the Islamic State, the militant organization that controls large swaths of Syria and Iraq and has become notorious for beheading hostages.