His employer Bob Marich told CBC News that McDonald borrowed his tools to dig the tunnel.
Marich saw the CBC News story about the tunnel, recognized the equipment and alerted the authorities, he said. He even attempted to call Mayor John Tory directly, but his call routinely was transferred to Toronto Water, he said.
Marich said he and McDonald had discussed offering interviews to the media for money, in order to help McDonald pay an $800 fine levied by the city and to replace the cost of the tools he borrowed and lost.
McDonald, a construction worker, lives close to where he tunnelled a nearly 10-metre-long underground chamber.
He described himself to the Sun as the "main digger" for the tunnel.
The paper quotes McDonald as saying it took about five months to build the bunker and that he had planned to add a few more rooms and put in a TV. He reportedly told the Sun the chamber had been there more than two years when police were alerted to its presence in January.
Toronto police said Monday that two men built the tunnel as a place to "hang out" and that no criminal charges would be laid. The discovery was announced a week earlier, prompting widespread speculation about the chamber's origin and purpose.