Mohamed Jalloh, 26, of Sterling, Va., is expected to make an initial appearance Tuesday afternoon in federal court in Alexandria.
A court affidavit spells out a three-month sting operation in which Jalloh said he was thinking about carrying out an attack similar to the 2009 shootings at Fort Hood, which left 13 people dead.
Jalloh's case is the most recent of several in which men from the northern Virginia area just outside Washington have been charged with attempting to support the Islamic State group.
Court records indicate the FBI saw Jalloh buying an assault rifle Saturday at a gun shop in Chantilly. The affidavit is not clear as to whether authorities believe Jalloh planned to use the rifle himself or whether he may have been procuring it on behalf of an informant. He was arrested Sunday.
Jalloh's sister, Fatmatu Jalloh, said in a brief telephone interview that she is serving as one of her brother's attorneys. She said she had not yet seen the unsealed charges but denied he would be helping the Islamic State group.
Jalloh is identified in the affidavit as a naturalized U.S. citizen originally from Sierra Leone.
The affidavit says the investigation began in late March, when an unidentified member of the Islamic State group brokered an introduction between Jalloh and a government informant. According to the affidavit, Jalloh had been communicating with the Islamic State member, who is now deceased, before he ever interacted with the government informant. The FBI agent says in the affidavit that he believes Jalloh met the Islamic State member on a trip to Africa early this year.
In April, Jalloh told the informant he had been thinking about carrying out a Fort Hood-style attack. Asked to explain, Jalloh said, "Nidal Hasan type of things. That's the kind of stuff I started thinking," according to the affidavit.
Later in April, Jalloh told the informant that the Islamic State group had asked Jalloh if he wanted to participate in an attack. According to the affidavit, Jalloh told the Islamic State representative that "I really want to but I don't want to give my word and not fulfill it."
In May, according to the affidavit, Jalloh tried to give a $500 donation to the Islamic State, but the money he sent actually went to an account controlled by the FBI.
Last month, the affidavit says, Jalloh drove to the Charlotte, N.C., area with another person looking for weapons to buy.
On Friday, Jalloh tried to buy an AR-15 assault rifle from a Chantilly gun store but was turned away because he lacked the proper paperwork. The affidavit says he returned the next day and bought a different assault rifle, which was rendered inoperable before he left the store.
Jalloh's arrest comes on the heels of several other cases in the area. Mohamad Khweis of Alexandria was charged after traveling to join the Islamic State group in Iraq and Syria, then surrendering himself to Kurdish forces after becoming disillusioned with the group. Two Woodbridge men, Mahmoud A.M. Elhassan and Joseph Farrokh, were charged with trying to join the Islamic State. Farrokh has pleaded guilty, while Elhassan awaits trial.
Last year, 17-year-old Ali Shukri Amin of Manassas was sentenced to more than 11 years in prison for helping another teen travel to Syria to join the Islamic State.