London’s Metropolitan Police Service, which had jurisdiction over the case, said in a statement that Mr. Miranda had been lawfully detained under the Terrorism Act and later released, without going into detail. “Holding and properly using intelligence gained from such stops is a key part of fighting crime, pursuing offenders and protecting the public,” the statement said.

The Guardian published a report on Mr. Miranda’s detainment on Sunday afternoon.

Mr. Greenwald said someone who identified himself as a security official from Heathrow Airport called him early on Sunday and informed him that Mr. Miranda had been detained, at that point for three hours. The British authorities, he said, told Mr. Miranda that they would obtain permission from a judge to arrest him for 48 hours, but he was released at the end of the nine hours, around 1 p.m. Eastern time.

Mr. Miranda was in Berlin to deliver documents related to Mr. Greenwald’s investigation into government surveillance to Ms. Poitras, Mr. Greenwald said. Ms. Poitras, in turn, gave Mr. Miranda different documents to pass to Mr. Greenwald. Those documents, which were stored on encrypted thumb drives, were confiscated by airport security, Mr. Greenwald said. All of the documents came from the trove of materials provided to the two journalists by Mr. Snowden. The British authorities seized all of his electronic media — including video games, DVDs and data storage devices — and did not return them, Mr. Greenwald said.

A spokesman for the British Foreign Ministry said the episode was a “police matter” and would provide no further comment. Civil rights groups in Britain have criticized Section 7 of the Terrorism Act, accusing the authorities of using the provision to arbitrarily stop and detain travelers, particularly Muslims. The British Home Office has said it is reviewing the provision in an effort to address the concerns.