The sign on the front of the modern, one-story stone and brick building reads: "Holy Land Foundation Relief & Development" and, beneath that, "A Helping Hand for All Mankind."

Today, another sign was posted, this one on the front door: "These premises have been sealed by order of the U.S. Treasury Department, office of Foreign Assets Control."

Even as FBI agents and U.S. Customs Service officials watched workers lug computers and boxes of documents from Holy Land's office park headquarters into a 14-wheel tractor-trailer, the group's defenders denied U.S. accusations that it is affiliated with the Palestinian extremist movement Hamas. Instead, they said, it is a main source of humanitarian aid to bedraggled Palestinians requiring medical care, food and schooling.

"Holy Land has been the only help for many in the Palestinian population in the West Bank and Gaza, and now, with this move, a big void will exist," said Khalid Hamideh, a Texas attorney for the group.

Federal government officials acknowledge that Holy Land, founded in 1989, is a Muslim charity that raised $13 million last year for Islamic causes around the world. But after investigating the agency for years, they also say that it is intimately connected with Hamas, an organization that has killed scores of Israelis and some Americans in suicide bombings in Israel over the last decade.

Holy Land "acts for or on behalf of Hamas" in the United States, the FBI said in a statement today, as federal officials went further than ever before in outlining Holy Land's alleged Hamas ties.

The FBI said, for example, that Shukri Abu Baker, Holy Land's founder and now chief executive, "has been repeatedly identified as a member [of] Hamas" -- an assertion Holy Land officials denied. The bureau said an informant quoted him as saying the group's "mission" is financially supporting the families of suicide bombers.

Moreover, a ranking Hamas political operative named Mousa Abu Marzook gave $200,000 to Holy Land in 1992, the bureau statement said, and two years later, he privately designated Holy Land in Muslim circles as Hamas's "primary fundraising entity in the United States, and ordered a rival fund to curtail operations."

Holy Land Executive Director Haitham Maghawri once told the U.S. Immigration and Naturalization Service that he had been arrested for planting a car bomb in a foreign country, the FBI said.

Representatives of Holy Land and an array of Islamic groups yesterday repeated their response that the allegations against the organization are falsehoods fabricated by the Israelis.

"The attorney general's office has been looking into Holy Land since probably 1992," said Ghassan Elashi, Holy Land's chairman. "They have overstepped. . . . All these allegations are false. They are absolutely coming from Israel."

Foundation records show that, in some cases, Palestinian families tied to Hamas receive much larger sums from Holy Land than those lacking such affiliations, the FBI said.

One of the most incendiary allegations lodged by President Bush and other government officials against Holy Land is that it encourages terrorist attacks by supporting the families of Hamas suicide bombers and jailed Hamas militants. The foundation "assists Hamas by providing a constant flow of suicide volunteers and buttresses a terrorist infrastructure heavily reliant on moral support of the Palestinian populace," the FBI statement said. It cited several such cases, such as Holy Land aid to the family of a Hamas member in jail for killing a Canadian Jewish tourist on a Tel Aviv beach.

Holy Land officials maintained today that such payments constitute a tiny portion of the money the group spends supporting orphans and destitute families in the West Bank and Gaza.

Elashi, who is related by marriage to Hamas official Marzook, also scoffed at one of the key allegations lodged by Israeli officials for years -- that the schools for Palestinian children funded by Holy Land glorify suicide bombing and try to recruit martyrs for their movement.

"The Palestinian children are beautiful children, and we are helping them become suicide bombers?" Elashi said. "No, no, no. It is unbelievable to think that."

The FBI said Holy Land provides funds to Hamas in the West Bank and Gaza in three ways: through its own offices in the territories; through other Muslim charities controlled by Hamas; and through other charities not directly run by Hamas but supporting it.

At a 1993 meeting in Philadelphia, the FBI said, top Hamas and Holy Land officials agreed that Muslims in this country could be key to financing Hamas.

"In the United States, they could raise funds, propagate their political goals, affect public opinion and influence decision-making of the U.S. government," the FBI said the participants agreed.

"The democratic environment in the United States allowed them to perform activities that are extremely important to their cause," the FBI quoted them as saying. "In discussing financial matters the participants stated a belief that continuation of the Holy War was inevitable."

Staff writer Kathleen Day contributed to this report. She and Mintz reported from Washington.

Shukri Abu Baker, Holy Land's founder and chief executive, conducts an interview. The FBI says he has been identified as a member of Hamas.