In 1980, the conceptual artist Allan Bridge came up with an idea to allow strangers to leave confessions on a telephone answering machine. He distributed posters and flyers around Manhattan’s Tribeca district which read: “Attention amateurs, professionals, criminals, blue collar, white collar. Get your misdeeds off your chest!” Callers were provided with a number and advised not to leave identifying details, since Bridge planned to make the recordings public. His answering machine quickly filled up.

There were apologies from cheating spouses, child runaways, and a woman who felt guilty about being rich. A white man apologised to the black, Puerto Rican and Chinese communities in his neighbourhood for having racist thoughts; another who witnessed a robbery expressed regret for not reporting it. There were dark confessions of theft and violence. One man wanted to apologise to “the 15 or 20 people I’ve stolen money from, and mugged and robbed and frightened”. Then he said he had murdered one of his neighbours.

The story of Allan Bridge, his callers and the ethical complexities of the art project that would consume him for 15 years is told in Wondery’s The Apology Line. The series would not exist had Marissa Bridge, who is Allan’s widow and the podcast’s narrator, not kept the hundreds of tapes gathered by her husband. We hear how the recordings were first made available to the public in a New York art gallery; visitors would go into individual phone booths and listen through the receiver. Clips were subsequently played on radio shows, and transcribed in magazines. For a while, Bridge — who kept his identity secret and became known as “Mr Apology” — put a selection of them on his answering machine, allowing callers to hear them before leaving their own messages.

Marissa’s delivery is on the wooden side, but no matter since the recordings alone make for remarkable listening. While some are tinged with sadness and shame, others carry the sound of a weight being lifted. A handful are downright chilling, and it becomes clear that what started out as art with altruistic intent became a repository for the darkest human impulses, prompting inevitable interest from police (after much agonising, Allan Bridge allowed detectives to listen to the recordings only when they had been publicly aired). Criminal callers went by pseudonyms such as Hard Time, Mr D and Candy Ray. A man called claiming to be the Zodiac killer.

One long and rambling message left me especially queasy. The caller starts out by giving thanks for a “brilliant” phone line that absolves callers of their crimes and “leaves no more guilt, no more problems . . . Oh boy, this is absolutely fantastic.” Then he adds: “[To] the person who is running this service . . . I will find out who you are and I’m telling you right now, I’m sorry, but I’m going to kill you.” Bridge pressed on with his project regardless.