Tree identification by roots

Tree roots blocked storm sewer line under driveway.
Any help identifying culprit would be appreciated. Roots apparently 7 - 9 feet down.
Nearby suspects are 1)very large weeping willow, 20 ft from blockage (tree has 25ft diameter canopy) 2) tall pyramid cedar, 10 feet from blockage 3)low junipers, 10 feet from blockage.

 

get samples from the known trees/shrubs and compare.

 

Thanks, but was hoping for a simpler solution. The trees are next to a driveway and the paved road; extremely difficult to dig to get samples.

 

I would suspect the weeping willow. Their roots seek out water vigourously