Prosecutors have said that Ms. Salman made financial arrangements with Mr. Mateen in advance of the attack and fabricated a cover story for her husband hours before the shooting. The F.B.I. said in statements to the court that she later confessed to agents that she had joined Mr. Mateen on a trip to scout the nightclub as a possible target.

On the night of the shooting, according to court records, Ms. Salman told Mr. Mateen’s mother that her son planned to have dinner with a friend, identified in court records only as Nemo.

Prosecutors point to this as indicating Ms. Salman’s complicity with her husband’s plot. But her lawyers say that Ms. Salman was in the dark and believed her husband did indeed have plans with Nemo, who lived in Baltimore. Investigators said Nemo told them that he had served as Mr. Mateen’s cover story to his wife when he cheated on her.

As part of her confession, according to the F.B.I., Ms. Salman said she and Mr. Mateen drove past Pulse while they were in Orlando a few weeks before the massacre. But court records filed by prosecutors about the locations Mr. Mateen cased in advance of the attack do not include the nightclub, raising doubts about Ms. Salman’s statements to the F.B.I. — and whether they might have constituted a false confession.

Ms. Salman, who holds an associate degree in medical administration, is not expected to testify at the trial, which will probably last about a month. The proceedings began on Thursday in Orlando with jury selection before Judge Paul G. Byron of the United States District Court for the Middle District of Florida. At least 75 Pulse victims have told prosecutors they are interested in attending at least portions of the trial.

Three years before the shooting, in 2013, the F.B.I. investigated Mr. Mateen after he told colleagues he was a member of Hezbollah, had family connections to Al Qaeda and wanted to die a martyr. That investigation was closed after 10 months, though Mr. Mateen was questioned in another investigation that year.