Weakening Evidence?
I doubt the excuse given is true. From what I have seen about criminal cases, either you have a pretty solid forensic case or you don't. If you have solids forensic case it does not suddenly weaken with time unless you are incompetent such as you lost all traces of the test results or your lab 'dry labbed' the results (aka faked them). Now if they had a weak forensic case they were relying on a confession to get a conviction. However, if the interview is done incorrectly the 'confession' can be challenged as being forced under duress. The movie 'Boomerang' was based on 1920's murder case were there was a forced confession and other shaky evidence. The DA in this case took 90 minutes in court to outline why he refused to prosecute the defendant, indicating the defendant was innocent.