For U.S. Forces, a Secret and Futile Hunt

But the mood turned serious at nightfall, when they made their way across the porous border into Pakistan. Eight Americans dressed in camouflage uniforms stripped of any insignia and wearing bulletproof body armor were accompanied by Mustafa, 38, his interpreter Shafiq, 21, and a fighter in his late thirties named Hafar. As they crossed the border, the Americans pulled dark masks down to their chins and put on their night-vision goggles.

The Pentagon had no comment on the operation. But judging by the Afghans' description, the Americans may have been members of Delta Force, the most celebrated and highly trained of the U.S. Special Forces. Unlike regular soldiers, these Americans did not have crew cuts and some sported beards, demonstrating the greater latitude allowed Delta.

They walked from the checkpoint in the dark for an hour before reaching the Pakistani village. The U.S. group made its way to a house where Mustafa's two Pakistani contacts had said Zawahiri and Haqqani were staying. The guides recalled that the small village of houses and a general store had no electricity, leaving the night completely dark except for a few kerosene lamps burning in some houses. Roaming around the forests nearby were jackals and foxes, Mustafa said.

Along the way, the Americans asked a steady stream of questions -- what's the name of this place, what is that called, how many people live here -- while carefully writing down the answers and keeping in contact with their compatriots by radios or satellite telephones, the guides recalled. Shafiq, dressed in thin clothes and unlike the others not carrying a gun, hugged himself to keep warm.

Mustafa said that, as Afghans, they had no choice but to sneak around, even without the U.S. soldiers. They would be immediately noticed, he added, although they shared a language with residents of that part of Pakistan. "We could not go openly around there because there is a lot of difference, and the dialect of their Pashtu [language] is different than ours," he said. "We didn't want to get in trouble. So we went only at night."

The house they scouted appeared calm and ordinary, the guides recalled. By 10 p.m., the lamp inside went out, but rather than approach too closely, Mustafa said the U.S. soldiers sent the pair of Pakistani contacts -- wired with sophisticated communications equipment that he would not describe. They determined that a family was living inside and found no sign of their quarry.

They planted themselves for the night nonetheless, in case Zawahiri or Haqqani came back.

While they waited, they pulled out knives and opened a few packages of military rations, known as Meals Ready to Eat, or MREs.

The night passed slowly and uneventfully, the guides recalled. Before dawn, the expedition packed up to go, remembering to pick up the MRE wrappings, lest they reveal their presence.

They made their way back to the checkpoint on foot as the sun was beginning to creep above the horizon. There they slept and ate, and headed out again the next night, with the same results.

After two nights, Mustafa had to return to Khost to deal with rising local factional tensions. But the Americans went back each night for the rest of the week with another translator and, as far as Mustafa and Shafiq know, continued to come up empty.

If Zawahiri and Haqqani were ever there, they evidently had moved on to their next safe house. Mustafa said he believes the information he received was correct. "They were in the village," he said, but had left by the time their hunters arrived.