National Transportation Safety Board investigators have told 7NEWS that the problem of cracked airplane windshields at Denver International Airport is more extensive than first thought.Initial reports indicated there were about a dozen planes that were damaged over a 90-minute period last Friday. 7NEWS has learned that the windshields on 20 planes were damaged.No one was injured, but the planes returned to the terminal at DIA.Cold weather and gusting winds are considered possible causes, but investigators say that is only a theory at this point. Glass specialists will arrive in Denver next week to help with the investigation.ABC News Aviation Consultant John Nance told 7NEWS that it is important for investigators to find a cause."We have a number of situations over the past 20 to 30 years where little anomalies were ignored that turned out to be very big things that lead to an accident," Nance said.Nance said the cracked windows are not an obvious safety hazard, but said they are very expensive to fix. The airlines involved have also lost money because of canceled flights. Frontier, Sky West and Great Lakes Airlines all suffered cracks.