DETROIT (Reuters) -- U.S. safety regulators closed a four-year probe into power steering problems in almost 335,000 older Saturn Ions after General Motors Co recalled the cars, which are also part of an unrelated recall of defective ignition switches linked to at least 13 deaths.

The U.S. National Highway Traffic Safety Administration, in documents filed in its online database over the weekend, said it was closing its probe into 2004 to 2007 model-year Ion cars after GM's recall of them last month. The agency initially opened a preliminary evaluation of the issue in January 2010.

GM has been criticized for not acting fast enough on the consumer complaints surrounding recalls for the power steering issue as well as the defective switches linked to at least 13 deaths and the recall of 2.6 million vehicles.

The Ion was among the 1.5 million cars GM recalled on March 31 for possible loss of power steering. Three of the six models recalled, including the Chevrolet Cobalt and HHR, are also involved in the ignition-switch recall.

On March 31, when GM announced the power-steering recall, the company's global vehicle safety chief, Jeff Boyer, acknowledged the company had previously recalled other cars with the same issue and GM "did not do enough."

NHTSA also has come under fire for not pushing GM to recall cars affected in the switch and power steering issues. In the Ion power steering recall, NHTSA said it found 4,787 complaints and 30,560 warranty claims related to the issue, resulting in a complaint rate of 14.3 incidents per thousand vehicles and a warranty claim rate of 9.1 percent.

There were 12 crashes related to the Ion steering failure, two resulting in injuries to the drivers, according to the NHTSA documents.

Three of the crashes reported occurred in parking lots, six occurred while turning at street intersections and one on a highway exit ramp, according to the NHTSA documents. The crashes in parking lots or intersections all occurred at speeds of less than 30 miles per hour, while the incident on the highway exit ramp involved impact into a guard rail at about 35 mph.

GM said if power steering assist is lost, a message is displayed on the dash and a chime sounds to inform the driver.

"GM indicated that steering control can be maintained in manual (unassisted) steering mode, but would require greater driver effort at low vehicle speeds, which could increase the risk of a crash," according to the NHTSA documents.

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