WASHINGTON — A young college student from Saudi Arabia studying chemical engineering in Texas purchased explosive chemicals over the Internet as part of a plan to hide bomb materials inside dolls and baby carriages to blow up dams, nuclear plants or the Dallas home of former President George W. Bush, the Justice Department said Thursday.

“It is war … until the infidels leave defeated,” the student wrote in online postings.

One of the chemical companies, Carolina Biological Supply of Burlington, N.C., reported suspicious purchases by Khalid Ali-M Aldawsari, 20, of Lubbock, Texas, to the FBI on Feb. 1. Within weeks, federal agents had traced his 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.

Aldawsari, who was legally in the U.S. on a student visa, was expected to appear in federal court on Friday. He was charged Thursday with attempted use of a weapon of mass destruction. Aldawsari entered the U.S. in October 2008 from Riyadh, Saudi Arabia, to study chemical engineering at Texas Tech University, then transferred earlier this year to nearby South Plains College.

The terrorism case against Aldawsari was significant because it demonstrated that radicalized foreigners can live quietly in the U.S. heartland 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.

The White House said President Barack Obama was notified about the plot prior to Aldawsari’s arrest Wednesday. “This arrest once again underscores the necessity of remaining vigilant against terrorism here and abroad,” White House spokesman Nick Shapiro said in a statement Thursday.

In e-mails Aldawsari apparently sent himself, he listed the names of 12 reservoir dams in Colorado and California. He also wrote an e-mail that mentioned “Tyrant’s House” with the address of President Bush’s home. The FBI’s affidavit said he considered using infant dolls to hide explosives and was possibly targeting a nightclub with a backpack filled with explosives.

The FBI said the North Carolina company reported the attempts to purchase phenol, a chemical that can be used to make the explosive trinitrophenol, also known as TNP, or picric acid. Aldawsari falsely told the supplier he was associated with a university and wanted the phenol for “off-campus, personal research,” according to court records. But frustrated by questions, Aldawsari canceled his order and later e-mailed himself instructions for producing phenol. Prosecutors said that in December 2010, he successfully purchased concentrated nitric and sulfuric acids that are combined to make TNP