I was pleased to see that the Denver International Airport, United Airlines, Avis and Supershuttle all have made special arrangement to accommodate people who need to get to the Denver area in the wake of the Columbine High School shootings.
United have very generously set up special discounted, fully-refundable fares from over 100 cities into Denver, with a special telephone number. The fares are good through 31st May.
Avis, Supershuttle and the airport have all taken steps to accommodate the people as well. I did not see anything from Frontier on their site, but I didn't check further, so maybe they just didn't post it.
Kudos to all of them for being so nice.