Seattle has a public works mystery. In less than a month, two different sewer lines were found to be blocked by concrete, causing at least $350,000 in repairs, according to multiple reports.

The first incident occurred on Fairview Avenue North, clogging 70 feet of the pipe, reported KIRO 7.

"We don't know who did it. We're fairly certain we'll find out who did it because that amount of concrete is probably coming from a construction site nearby," Cornell Amaya, with Seattle Public Utilities told the station.

The second incident will likely delay completion of Aurora Avenue North construction, reported The Seattle Times.

Work on Aurora was supposed to be finished Friday, until crews found the blocked and collapsed segment of pipe. That will send the job into overtime — up to two weeks longer, SPU spokesman Andy Ryan told the paper.

Samples from both incidents will be submitted to a laboratory for testing, to determine the origin of the concrete.

Describing the testing, KOMO reports:

A large contraption called a Universal Testing Machine makes a deafening noise as it presses up to 400,000 pounds of pressure per square inch, crushing a concrete core sample into pieces. Their trained staff can make visual assessments of concrete and look at the microscopic ingredients in the concrete to determine who made it and where it was used.

Want to know more? Check out this KIRO video.