MAKHACHKALA, Russia—The mother of the two young men alleged to have plotted and carried out the Boston Marathon bombing was placed on the same classified watch list as her elder son, according to U.S. officials, raising further questions about her role in his apparent radicalization.

Zubeidat Tsarnaeva and her son Tamerlan were placed at the same time in late 2011 on the Terror Identities Datamart Environment database, a low-level watch list that contains the names of more than 500,000 people flagged by multiple U.S. security...