The National Safety Transportation Board has found that helicopter traffic out of Ronald Reagan International Airport posed an “intolerable risk to aviation safety by increasing the chances of a mid-air collision” ahead of a crash last month that resulted in the deaths of 67 people.

When Congress pushed ahead last year with adding 10 new daily flights to Washington D.C.’s Reagan National Airport, the Federal Aviation Administration had data showing an unnerving number of near misses in the already-crowded skies — something lawmakers apparently did not know.