Midlothian is just a small country town. I know that Arlington PD was assisting with organizing all the tips that come in. They have more experience at this (Arlington is where the namesake for the Amber alert - Amber Hagerman - was abducted in 1996) & they developed a system at that time for entering/organizing/tracking tips. Texas Rangers usually get involved if it is a high-profile crime - which this definitely seems to be due to location (Midlothian hasn't had a homicide in like 9 yrs,) the victim & the sheer brutality of the murder. Law Enforcement probably called upon the assistance of the FBI to help them investigate simply because they have more resources. ATF assists in many violent crime investigations - simply to look for evidence of firearms, ammunition, arson, explosives, etc.
Thanks for the explanation.
