While the city of Bellevue claimed back in March it could take as long as a month to know what caused a sinkhole to open up in downtown Bellevue, public records show the city knew construction was to blame from the start.

Bellevue Transportation spokeswoman Tresa Berg stated ­in a March 17 email ­the city did not have an exact cause for why a sinkhole opened up two days earlier on the 10500 block of Northeast Fourth Street, causing one vehicle to become stuck.

She further reported Sellen Construction was investigating what caused the 4-­foot ­deep sinkhole, though heavy rainfall that day was being considered a potential factor. Construction was named as another.

According to an email from Bellevue Surface Water Superintendent Don McQuilliams to utilities staff members on March 15 —­ obtained by the Reporter through a public records request —­ the sinkhole was the result of a contractor leaving filter sacks in catch basins, “causing the water to undermine a patch in the roadway.”

Sellen Construction is the general contractor for Schnitzer West’s 425 Centre tower at 415 106th Ave. N.E., at the corner of Northeast Fourth, near where the sinkhole formed.

A supervisor for Sellen Construction said he believed he knew what caused the sinkhole ­— when contacted by the Bellevue Reporter last month ­— but declined to comment further because he was not permitted to speak to media. Neither Sellen Construction CEO Bob McCleskey nor president Scott Redman returned calls for comment when contacted about the sinkhole investigation last month.

The 63­-year-­old Renton man whose Toyota Corolla ended up stuck in the sinkhole told police he saw what he thought was a small pothole in standing water on Northeast Fourth, but could not move around it due to traffic, according to police records. The man was uninjured, and his car was towed out of the hole.

The contractor responded March 15 by correcting the drainage problem it caused and also using steel plates to mitigate the sinkhole.