ISLAMABAD -- The Islamist militant group behind the deadly attack in Mumbai one year ago remains a potent force determined to strike India and the West, and a source of acrimony between South Asia's nuclear-armed rivals, say officials and members of the militant faction.

Indian officials and experts say at least six new plots against Mumbai by the Pakistan-based group, Lashkar-e-Taiba, have been disrupted in the 12 months since 10 gunmen wrought three days of havoc on India's financial capital, killing 166 people.

Lashkar's...