BISMARCK, N.D. (AP) — Bolt failures caused a wind turbine's rotor and blades to fall from a tower in north-central North Dakota, and six other turbines have been shut down while their bolts are replaced, a state regulator said Thursday.

Members of North Dakota's Public Service Commission, which oversaw the development of the 71-turbine wind farm, said Thursday they would seek more detailed information about how widespread the problems may be.

"That's a fair bit of equipment concern that I would have, quite frankly," Commissioner Kevin Cramer said.

The wind project, which was dedicated last October, is located near Rugby in Pierce County. It is capable of generating up to 149 megawatts of electricity.

It is owned by Iberdrola Renewables Inc. of Portland, Ore., which is a unit of Iberdrola Renovables SA of Valencia, Spain. The turbines themselves were manufactured by Suzlon Wind Energy Corp., a unit of Suzlon Energy Ltd., based in India.

Spokeswomen for Iberdrola and Suzlon did not immediately reply to telephone and email requests for comment Thursday. Suzlon has previously described the accident as an isolated incident.

Jerry Lein, a commission utility analyst, said Iberdrola officials told him that bolts that attached the wind turbine's rotor and blades to a power shaft had failed. The shaft transfers the energy generated by the turning blades to an electric generator.

No one was injured when the rotor and blades toppled from the tower March 14 and crashed to the ground.

Lein said the wind farm was shut down and its turbines inspected. The turbines that did not need bolt replacement have been restarted, he said. The damaged material has been sent to a lab for analysis.

"They want to look further into the mechanism there that was failed before," he said. "They said that, specifically, they're replacing the bolts that hold it together."

The bolts are normally checked every six months, Lein said.

Commissioner Brian Kalk said the agency should seek to examine the wind farm's maintenance records. He wants to hear more information from the companies within two weeks, Kalk said.

"I'd like (the companies) to get back in front of us as quickly as possible ... and give us their best estimate of what is going on," Kalk said.

The commission's president, Tony Clark, said the agency should hold an informal hearing on the incident.

"Nobody has a greater incentive to find out what went wrong than the company does," Cramer said. "But, at the same time, the citizens of Pierce County, they're probably a little bit concerned too. ... You might not want to go hiking in the prairie for a while."