Jose Padilla, the American citizen accused of plotting to detonate a radioactive "dirty" bomb in the United States, traveled to a central Asian country in April looking to buy radioactive materials, Pakistani intelligence and government officials said Thursday.

The officials, who spoke on condition of anonymity, said at least two associates of Padilla are now in custody in Pakistan, and FBI agents are questioning them at an undisclosed location. But the officials would not give the associates' nationalities.

The officials said at least a half dozen American citizens were believed to be among the 300 Al Qaeda suspects handed over to the United States by Pakistan in the past six months.

Maj. Gen. Rashed Quareshi, Pakistan's Director of Interservices Public Relations, told Fox News that two Afghan-born American citizens were arrested for illegal border crossing Wednesday in the tribal area of Western Pakistan.  Quareshi said there was no evidence that the two were connected to Al Qaeda, but he said the investigation was continuing.

As far as other people of "American origin" or other Americans arrested by Pakistani authorities, the General said those are the only Americans he knows of who have been captured.

He said a British national who had been with Padilla is currently in custody. Fox has previously reported that one of Pedilla's associates is named Benjamin Ahmed Mohammad, but Quareshi said he could not confirm that.

The hunt for the Brooklyn-born Padilla, 31 also known as Abdullah al Muhajir, began after the arrest in March of Usama bin Laden's lieutenant Abu Zubaydah, the officials said. Zubaydah is currently in U.S. custody.

Padilla had apparently been assigned by Zubaydah to explore the possibility of causing "maximum damage to Americans," intelligence officials said, also on condition of anonymity.

"We had received information from the FBI in April that some Al Qaeda men were planning attacks in the United States. The same information was shared with us by the CIA and intense weeks-long efforts led to his arrest," said a senior government official, who spoke under condition of anonymity.

Authorities began to track Padilla in early April after the FBI provided photos of the former Chicago gang member to Pakistan's spy agency, Inter-Services Intelligence, the officials said.
Using false documents and aliases, Padilla traveled to an undisclosed central Asian country in April, the officials said. They would not say how long he remained in central Asia nor whom he contacted.

The officials said Padilla returned to Pakistan and left Karachi in late April or early May for Zurich, Switzerland. U.S. authorities have said Padilla made trips from Switzerland to and from Egypt before flying to Chicago, where he was arrested May 8.

Pakistani officials are expanding their search for more Padilla associates in the deeply conservative tribal areas along the Afghan border, where many Al Qaeda fugitives are believed to have taken refuge.

The FBI is particularly interested in finding five Arab women, seven children and several Al Qaeda men believed to have close contacts with Zubaydah and Padilla.

Pakistani sources, speaking on condition of anonymity, said U.S. and Pakistani agents raided an Islamic cleric's home in the North West Frontier Province this week looking for the women and children but found no one.

Fox News' Bret Baier and the Associated Press contributed to this report.