US authorities have arrested a 15-year-old for allegedly plotting an Islamic State-inspired attack that US media said had Pope Francis as its target.
Francis is scheduled to visit the United States for the first time, in a tour culminating with an outdoor mass in Philadelphia after stops in Washington and New York.
The US Department of Homeland Security and the FBI disclosed the unidentified 15-year-old's arrest in a joint intelligence bulletin on August 14, without mentioning that the Pope was the target.
"The minor was inspired by ISIL and sought to conduct a detailed homeland attack which included multiple attackers, firearms, and multiple explosives, targeting a foreign dignitary at a high-profile event," it said.
"The minor obtained explosives instructions and further disseminated these instructions through social media."
ABC News cited unnamed sources as saying the "foreign dignitary" referred to in last month's statement was Pope Francis, and that the boy was arrested near Philadelphia.
ISIL is an alternate acronym for Islamic State, the extremist group engaged in atrocity-filled campaigns to establish a "caliphate" in Syria and Iraq.
The papal visit poses enormous security challenges for US authorities tasked with protecting a religious leader known for his popular touch and love of crowds.
In Philadelphia, two million people are expected to turn out for an open air Sunday mass outside the Philadelphia Museum of Art, at the close of an event billed by the Vatican as a World Meeting of Families.
Michael McCaul, chairman of the House Committee on Homeland Security, said on Sunday (local time) that US authorities had broken up a threat against the Pope, but gave no details.
"We're monitoring very closely threats against the pope as he comes into the United States. We have disrupted one particular case," McCaul told ABC's This Week show.
The joint FBI-Homeland Security statement cited the case of the 15-year-old as an example of how "some youth are vulnerable to messaging from ISIL".
But ABC said its sources had said the boy did not pose an imminent threat, his plans were "aspirational" and there were questions about his mental health.
Nevertheless, he was charged with attempting to provide material support to a designated foreign terrorist organisation, namely IS, the government said.