The police gave no indication of what they believed Mr. Lam’s motivation was.

Mr. Lam said that in early July, he wrote to F.C. Barcelona, the Spanish soccer club, to ask for a signed photograph of the Argentine star Lionel Messi to give to Liu Xiaobo, the imprisoned Chinese Nobel laureate.

Mr. Liu, who had late-stage liver cancer, died before Mr. Lam could give him the photo. He said that he received a call last week from a man speaking Mandarin, a language not much used among Hong Kong’s Cantonese speakers. The man warned him against giving the photo to Liu Xia, Mr. Liu’s widow, who remains under house arrest since her husband’s death.

Mr. Lam claimed that on Thursday, two Mandarin-speaking men approached him as he was shopping and forced him into a van. They knocked him out with a chemical that he was forced to inhale, he said, then took him to a rural area where they beat him and drove staples into his legs. He said he found himself on a remote beach early the next morning.

But FactWire, a local investigative news agency, compiled security video that appeared to show Mr. Lam heading home from shopping on his own, with no signs of a kidnapping.

Mr. Lam told the news agency that the time stamps on the videos were wrong and that he was not the person who appeared in most of the snippets wearing a hat and mask.