Investigators are looking into missteps by air-traffic controllers that may have contributed to the loss of radio contact with Northwest Flight 188 for more than an hour, according to people familiar with the matter.

Most of the public attention has focused on why the two experienced pilots aboard the cruising jetliner failed to respond to repeated radio calls from controllers and text messages from company dispatchers. Besides being out of radio contact during the flight from San Diego to Minneapolis-St.Paul, they overshot their destination by more than 100 miles before circling back and landing safely.

SLIDESHOW: Northwest Airlines Plane Mystery

Now, the National Transportation Safety Board has expanded its probe to look at apparent safety lapses by controllers, these people said. Investigators have scheduled interviews with controllers for next week.

Investigators are examining preliminary evidence that indicates that some controllers apparently were slow to recognize that the plane had lost contact. In addition, higher-level air-traffic control managers may have also failed to promptly follow the appropriate procedures to alert U.S. national-security agencies.

New details emerging about the errant flight suggest the incident began when the co-pilot acknowledged a Denver-area controller's routine instructions to switch radio frequencies, but never re-contacted controllers on the new frequency.

Click here to continue reading at the Wall Street Journal.