“We weren’t aliens,” Mr. Abdus-Salaam, 60, said in a telephone interview from Florida, where he moved in retirement. “We had a foothold there. You’d walk into the elevator in the morning and say, ‘Salaam aleikum,’ to one construction worker and five more guys in suits would answer, ‘Aleikum salaam.’ ”

One of those men in suits could have been Zafar Sareshwala, a financial executive for the Parsoli Corporation, who went to the prayer room while on business trips from his London office. He was introduced to it, he recently recalled, by a Manhattan investment banker who happened to be Jewish.

“It was so freeing and so calm,” Mr. Sareshwala, 47, said in a phone conversation from Mumbai, where he is now based. “It had the feel of a real mosque. And the best part is that you are in the epicenter of capitalism  New York City, the World Trade Center  and you had this island of spiritualism. I don’t think you could have that combination anywhere in the world.”

How, when and by whom the prayer room was begun remains unclear. Interviews this week with historians and building executives of the trade center came up empty. Many of the Port Authority’s leasing records were destroyed in the towers’ collapse. The imams of several Manhattan mosques whose members sometimes went to the prayer room knew nothing of its origins.

Yet the room’s existence is etched in the memories of participants like Mr. Abdus-Salaam and Mr. Sareshwala. Prof. John L. Esposito of Georgetown University, an expert in Islamic studies, briefly mentions the prayer room in his recent book “The Future of Islam.”

Moreover, the prayer room was not the only example of Muslim religious practice in or near the trade center. About three dozen Muslim staff members of Windows on the World, the restaurant atop the north tower, used a stairwell between the 106th and 107th floors for their daily prayers.

Without enough time to walk to the closest mosque  Masjid Manhattan on Warren Street, about four blocks away  the waiters, chefs, banquet managers and others would lay a tablecloth atop the concrete landing in the stairwell and flatten cardboard boxes from food deliveries to serve as prayer mats.