It's been three days since concrete was discovered in a city sewer near the James North GO station construction site, but Metrolinx still isn't sure how it got there.

Metrolix spokesperson Anne Marie Aikins said crews working on the GO Transit STATION near the McNab Street bridge were drilling on Monday. The next day, Aikins said they found a backed-up drain.

"Whether or not they went through the sewer (while drilling)... I don't know," said Aikins.

She did say that Metrolinx will be fixing the problem.

"We apologize for any inconvenience and are working with the City of Hamilton to remedy the situation as quickly as possible. The City of Hamilton will not be responsible for paying the costs associated with resolving this issue. There is a backup system in place to ensure that any sewer contents don’t go into the waterways," said Aikins.

The city did not immediately respond for comment Friday afternoon.