This article was reported by Eric Schmitt, Mark Mazzetti and Jane Perlez and written by Mr. Schmitt.

WASHINGTON — Lashkar-e-Taiba, the Pakistan-based militant group suspected of conducting the Mumbai attacks, has quietly gained strength in recent years with the help of Pakistan’s main spy service, assistance that has allowed the group to train and raise money while other militants have been under siege, American intelligence and counterterrorism officials say.

American officials say there is no hard evidence to link the spy service, the Directorate for Inter-Services Intelligence, or ISI, to the Mumbai attacks. But the ISI has shared intelligence with Lashkar and provided protection for it, the officials said, and investigators are focusing on one Lashkar leader they believe is a main liaison with the spy service and a mastermind of the attacks.

As a result of the assault on Mumbai, India’s financial hub, American counterterrorism and military officials say they are reassessing their view of Lashkar and believe it to be more capable and a greater threat than they had previously recognized.

“People are having to go back and relook at all the connections,” said one American counterterrorism official, who was among several officials who spoke on condition of anonymity because the investigation was still progressing.