premise: Also , no souvenirs of the crime , such as newspaper clippings , were found in his possession as commonly seen in serial murder cases . What 's more , the FBI could not place Ivins at the crime scene with evidence , such as gas station or other receipts , at the time the letters were mailed in September and October 2001 . While acknowledging the circumstantial nature of their case against Ivins , prosecutors argue they 're confident they would have been able to prove his guilt " beyond a reasonable doubt " in court . What 's a possible reason there were no souveneirs of the crime found in his possession ?
hypothesis: Because letters were mailed in September and October 2001 .