The FBI says send us $112 or we will lock your iCloud account.

Yes, it's a scam. The email supposedly from Service@FBI.gov and is titled, “Virus Warning: E-Mail from ‘FBI Alert”.

However, it is full of typos:

Virus Warning: E-mail from ‘FBI Alert’

Apple has detected an unauthorized sign-in to your iClodu [sic] account.

Please verify your account by sending 112$ to this Bitcoin address:

If no response is received your account will be locked for security.

The server will lock yor [sic] account within 2 hours if we don’t receive the payment!

We are working to create a world where privacy is the norm, end-to-end encryption is the standard, and security and usability are synonymous.

FBI and iCloud is selling a tool for iCloud protection against hackers and scammers this tool costs only 112$

the license for our tool is 360 days

if you are not familiar with bitcoin you can buy it from here:

coinbase(dot)com
bitstamp(dot)net
localbitcoin(dot)com

After we confirm the payment, we send the private key so you can unlock your email and download our tool

FBI SECURIRY [sic] iCloud and Apple Protection

The scammers are just throwing out this missive and seeing if it sticks to anything. Trash it if you receive such an email.

Source: MalwareBytes