The FBI didn't "allow" it. The DNC delivered the servers to the third party company and then refused to allow the FBI to inspect them. However, it does not appear that the FBI made any attempt to subpoena them to do an independent investigation.
As I posted on another thread about Comey:
Fidelity? Bravery? Integrity? COMEY?? The FBI used to command respect.....now it is an embarrassment of incompetence or actual criminality.
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...and yet Comey testified today
>> do you have any doubt that the Russian government was behind the intrusions in the DNC and DCCC systems and the subsequent leaks of that information? 00:19:57
Richard burr
>> no, no doubt. 00:19:58
James B. Comey Jr.