A Pennsylvania 19-year-old pledged allegiance to the Islamic State, used Twitter to spread its propaganda, and had a backpack with ammunition that suggests he might have been plotting an attack, federal authorities said Thursday.

Jalil Ibn Ameer Aziz of Harrisburg was arrested Thursday and charged with attempting to provide material support to terrorists. He was scheduled to have an initial appearance in federal court Thursday afternoon.

Court documents said a "go bag" or "tactical-style backpack" found in Aziz's closet contained a high-capacity magazine loaded with ammunition, a modified kitchen knife, a thumb drive, a tin filled with medicine, and a full-face mask similar to those worn by Islamic State militants. Aziz may have been "preparing to conduct or assist others in conducting an attack in the United States," according to a criminal complaint.

Aziz used Twitter to advocate violence against U.S. citizens and military members, and to disseminate ISIS propaganda, and also tried helping others travel to Syria to fight for ISIS, authorities said.

A message left at Aziz's home was not immediately returned Thursday. Court documents do not list an attorney who could comment on the charges.

"We will remain vigilant in our efforts to hold accountable any and all of those who seek to provide material support to terrorists, and to disrupt potential attacks in the United States before they happen," Assistant Attorney General John Carlin said in a statement.

Aziz "espouses violent jihadist beliefs and is a fervent supporter" of ISIS, according to the criminal complaint.

He has used 57 Twitter accounts - all traced to the home he shares with his parents - to advocate violence, encourage people to fund jihadist groups, and express a desire to travel to territory controlled by the Islamic State, the complaint said. Aziz would create a new account each time Twitter suspended him for violating its terms of service, court documents said.