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 .

Question: What 's a possible reason there were no souveneirs of the crime found in his possession ?
Answer: Because those are commonly seen in serial murder cases .Because the FBI could not place Ivins at the crime scene with evidence .None of the above choices .Because letters were mailed in September and October 2001 .