FBI agents spent a second day searching an unoccupied Palm Beach County home today as part of an investigation that law enforcement sources said is related to the Sept. 11, 2001, terrorist attacks.

Investigators decided to search the home because some of the Saudi nationals who lived there entered the United States at the same time as some of the Sept. 11 hijackers, and because most of the family left hastily two days before the attacks on the World Trade Center and Pentagon, the sources said.

Neighbors said they contacted the FBI shortly after the attacks to report their suspicions, and have grown more concerned because the family never returned. The home in Greenacres, a small community south of Palm Beach, is owned by Mohammed Almasri and Afuf Arif, according to Palm Beach County property records.

Next-door neighbor Martha Ruth said today that Almasri and his wife were a polite couple who knocked on her door to introduce themselves and their four children after buying the home in July 2000. Almasri often spoke about how proud he was that his son, Turki M. Almasri, now 22, attended flight school, Ruth said.

An FBI official said the son was enrolled at Huffman Aviation International in Venice, Fla., where Sept. 11 hijackers Mohamed Atta and Marwan Al-Shehhi studied.

The family spent summers in Florida and lived the rest of the year in Saudi Arabia, Ruth said. But she hasn't seen them since Sept. 9, 2001, when they hurriedly put luggage in a van and sped away after saying they were going back to Saudi Arabia.

Ruth's daughter, Kristina Daddio, said she and her siblings called the FBI at least a half-dozen times after the terrorist attacks to report their suspicions. Finally, Daddio said, she gave up trying.

When federal agents arrived Tuesday morning, Daddio said she vented her frustration. "I said, "What took you so long?' " she said.

What Daddio didn't know, law enforcement sources say, is that agents had entered the home in October under the authority of local laws that allow inspections of abandoned homes. The local homeowners association had contacted the FBI about the abandoned home and the family's quick departure, the sources said.

During the inspection, agents spotted something that gave them probable cause to request a search warrant, the sources said. They declined to discuss the item, and the search warrant is under seal.

Over the past two days, more than a dozen members of an FBI evidence team have pored over the house, digging at least two holes in the back yard and sifting through the dirt. Daddio said a car was towed away.

Some agents wore protective clothing, but Judy Orihuela, a spokeswoman for the FBI's Miami field office, said, "There is no public health issue. This is not anthrax."

Daddio said agents showed her several photographs, but she did not recognize the men pictured.

Eggen reported from Washington. Research editor Margot Williams contributed to this report.

Rodney Lamarca, center, a neighbor of the Almasri family, talks with an FBI agent and a local policeman.