Michael L. Levy, the United States attorney for Eastern Pennsylvania, said in a statement the case illustrated how terrorists were looking for American recruits who could blend in. “It shatters any lingering thought that we can spot a terrorist based on appearance,” he said.

Ms. LaRose is white, with blond hair and green eyes, according to the law enforcement official, who was not authorized to share details of the case and spoke only on the condition of anonymity. The official said Ms. LaRose was born in Michigan and later lived in Texas and Montgomery County, Pa.

The indictment said that in mid-2008, Ms. LaRose, using the aliases JihadJane and Fatima LaRose, began posting on YouTube and other Internet sites messages about her desire to help Muslims. A MySpace profile for a woman who refers to herself as JihadJane displays pictures of bloodshed and violence in the Middle East scrawled with messages like “Palestine We Are With You” and “Sympathize With Gaza.”

By early 2009, the court papers said, she was exchanging e-mail messages with unidentified co-conspirators in Southeast Asia and Europe and expressed a desire to become a martyr for an Islamist cause.

The indictment refers to e-mail messages in which a conspirator, citing how Ms. LaRose’s appearance and American passport would make it easier for her to operate undetected, allegedly directed her in March 2009 to go to Sweden to help carry out a murder. She agreed to do so, writing, “I will make this my goal till I achieve it or die trying,” the indictment says. She and another unnamed American later posted online solicitations for money for that project, the document said.

Ms. LaRose had attracted the government’s attention by then. She was questioned by F.B.I. agents on July 17, 2009, and falsely told them that she had never solicited money online for terrorism, had never used the alias JihadJane and had never made postings on a terrorist Web site, the court papers say.

Despite drawing the F.B.I.’s attention, the indictment says Ms. LaRose traveled to Europe in August, joined an online community hosted by the intended Swedish victim in September and performed online searches to track him. She apparently never attempted to carry out the killing.

The indictment also says Ms. LaRose recruited other people on the Internet to wage or support jihadist attacks.