Missoula, Montana (CNN) -- The defense and prosecution agree on this much: Jordan Linn Graham pushed her husband of eight days, and he fell off a cliff to his death in Glacier National Park in Montana.
The question for jurors will be whether Graham's act was murder or a case of self-defense that ended tragically.
The two sides set out their opening arguments Monday about what took place as Graham's trial began in U.S. District Court in Missoula, Montana.
Prosecutors said they would show that Graham, 21, was having serious second thoughts about her marriage before her husband's death and willfully lied to police after it.
But her defense lawyers said that the death plunge was an accident resulting from an argument. Graham initially lied to police, they said, because she was afraid she wouldn't be allowed to explain what happened on the cliff edge.
Her husband, Cody Johnson, disappeared July 7. Four days later, the FBI says, Graham led friends and relatives to a popular spot in the park, where they found Johnson's body.
The young bride at first maintained that she had simply speculated Johnson, 25, might have gone there. But an FBI agent said that she changed her story when she was shown a surveillance photo of the couple entering the park together.
What exactly Graham said next to the FBI will be fiercely contested at the trial.
At a pretrial hearing November 15, Graham testified, "We went on a little stump part and we were in the middle of an argument and he thought I was going to run away. Cody had grabbed me and I thought he was going to push me down. My first instinct was to get him off."
In a court filing, the defense said Graham pushed Johnson away as she removed his hand from her arm, and her husband tumbled over the cliff.
But the criminal complaint against her says that in an FBI interview, "Graham stated she could have just walked away, but due to her anger, she pushed Johnson with both hands in the back and as a result, he fell face first off the cliff."
Her attorney, federal public defender Michael Donahoe, has said the FBI did not record the first hour and 20 minutes of Graham's interrogation.
He accused an FBI agent of then making "an epic effort" to get Graham to use "key words" in a recorded session that would support a criminal conviction. A defense motion says that in two subsequent recorded FBI interviews, Graham said she acted in self-defense and that her husband's fall was an accident.
Graham, who had been a part-time nanny, is accused of murder and making false statements.
In court on Monday, prosecutors said they would draw on text messages that they say show Graham was unhappy in her marriage and testimony that tracks the formation of her actions to deceive police, including sending herself e-mails from a fake e-mail address.
The prosecution's evidence will "give you a window into Jordan Graham's mental state," said Kris McLean for the U.S. Attorney's Office.
Speaking for the defense, Andrew Nelson portrayed Graham as young, naive and socially inept, somebody whose own family describes her as withdrawn.
She wove a web of lies to police to "run from the reality of the situation -- instinctively, like a frightened rabbit," Nelson said.
Graham was scared that no one would let her explain what happened, he said, because everyone loved Johnson but no one liked her.
Nelson said Graham admits a degree of responsibility in her husband's death, which he described as the result of an "argument-grab-push-fall." But she is not guilty of first- or second-degree murder, he said.
The first prosecution witnesses in the case testified that Graham seemed unhappy in her marriage to Johnson.
The court also heard text messages that Graham sent around the time of Johnson's death that showed her panic and then her efforts to hide what had happened.
Kimberly Martinez, Graham's best friend and matron of honor at her wedding, said Graham had expressed regret just days after marrying Johnson.
"I should be happy and I'm just not," Graham wrote in a July 1 text message to Martinez.
In texts on July 5 and 6, Graham alluded to possible aggression by Johnson, telling Martinez that he had a temper.
Graham told Martinez that she would talk to Johnson about her concerns on July 7, the day of his death.
"If you don't hear from me at all again tonight, something happened," she said in text message.
That night, after Johnson had plunged to his death, Graham sent Martinez several panicked texts, saying she was "freaking out."
Jennifer Toren, a friend of Johnson, testified that Graham had lied to her in text messages the day after Johnson's death.
In one text, Graham asked Toren if she'd heard from Johnson "last night or today?"
"The last thing he said to me was that he was going for a drive with some friends that were visiting," Graham tells Toren.
The case is being prosecuted in federal court before U.S. District Judge Donald W. Molloy because the incident occurred in a national park.
CNN's Kyung Lah reported from Missoula, and David Simpson reported and wrote from Atlanta.