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Telephone numbers Mr. Aldawsari had provided to others were not working Thursday. No one answered the buzzer or a knock on the door at the address listed as Mr. Aldawsari’s apartment near the Texas Tech campus.

The case outlined in court documents is significant because it suggests radicalized foreigners can live quietly in the U.S. without raising suspicions from neighbors, classmates, teachers or others. But it also showed how quickly U.S. law enforcement can move when tipped that a terrorist plot may be unfolding.

“We think we have neutralized any other threats or imminent harm surrounding the actions that he’s charged with, but the investigation is continuing,” Mr. Casey said.

Mr. Aldawsari wrote that he was planning an attack even before coming to the U.S. on a scholarship, the court documents say. He said he was influenced by bin Laden’s speeches and he bemoaned the plight of Muslims.

Federal authorities said they learned of the plot after a chemical company, Carolina Biological Supply of Burlington, N.C., reported $435 in suspicious order by Mr. Aldawsari to the FBI on Feb. 1.

Separately, Con-way Freight, the shipping company, notified Lubbock police and the FBI the same day with similar suspicions because it appeared the order wasn’t intended for commercial use. Within weeks, federal agents had traced Mr. Aldawsari’s other online purchases, discovered extremist posts he made on the Internet and secretly searched his apartment, computer and e-mail accounts and read his diary, according to court records.

A Saudi industrial company, which was not identified in court documents, was paying Mr. Aldawsari’s tuition and living expenses in the U.S.

Mr. Casey declined to go into why the arrest occurred when it did.

“We just felt it was the right time,” he said.

Goldman reported from Washington.