Necessary for what? I dont see a Consitutional or practical reason for them to exist....or any other fed LE agency for that matter.
There are interstate crimes and crimes of a nature that require more of a centralized and able to operate in all jurisdictions agency.
Police agencies are set up to handle crimes within their own jurisdiction. They are not set up to handle national matters. They may be able to capture a guy from time to time that is wanted nationally, but they generally aren’t going to be out there investigating a national figure operating across state lines.
When it comes to a large operation like the OKC Bombing, you need to have a federal agency to fall back on. They need to collect information across state lines and even across U. S. borders.
It’s my take there does need to be a federal agency akin to the FBI.
BATF? No. Disband it today.