Ooh, here's a nice pdf:

https://www.socxfbi.org/common/Uploaded%20files/Membership/TaxDeductionForPSOs.pdf

"The distribution must be from a retirement plan maintained by the
employer from which the taxpayer retired as a public safety officer and
cannot be from some other retirement plan."

https://www.irs.gov/pub/irs-pdf/p721.pdf

Go to "Distributions Used To Pay Insurance
Premiums for Public Safety Officers"