What is experimentally verified is time dilation - this leads to other consequences such as the increase in mass with velocity, E= mc^2 etc. But there are several other theoies that lead to the same result e.g., LET, MLET, LR, Inertial Transforms and maybe some others. Its good to keep an open mind to the possibility that SR may not be the way nature works even though it has survived the tests to which it has been put. Near the end of his life Einstein expressed his own doubts.