What is known so far about why flights were grounded nationwide

Planes sit on the tarmac at Dallas Fort Worth International Airport in DFW Airport, Texas, Friday, Nov. ET Friday, allowing some domestic flights to take off from U.S. airports.

The longest flight stoppage in 25 years, lasting over four hours, ended around 8:15 p.m. Of the 41,000 flights scheduled Saturday, the Federal Aviation Administration expected as many as 5,000 will likely be canceled.