Washington regulators are investigating Waste Management's response to an eight-day Teamsters strike that disrupted garbage, yard waste and recycling collections for more than 200,000 customers in the Seattle-Everett area.

The state Utilities and Transportation Commission sent a letter to Waste Management's Kirkland office Thursday seeking a detailed status report on each day of the strike.

The commission says it's concerned the resumption of services was more protracted than the company's contingency plan indicated and that Waste Management may not have allocated replacement drivers equitably.

The commission says it will hold a public meeting Aug. 9 as it decides whether the company should be penalized.

Union drivers returned to work Thursday when members of Teamsters Local 117 approved a new six-year contract.