Are supposed to be things FBI and other agencies aren't supposed to be able to get.

These are supposed to be the two things you're supposed to have the most security over

NY State income tax documents include some portions of federal income tax filings as part of the required data... plus

IRC 6103(d) provides that return information may be shared with state agencies responsible for tax administration.
The state agency must request this information in writing, and the request must be signed by an official designated to request tax information.

And

IRC 6103(i)(1) provides that, pursuant to court order, return information may be shared with law enforcement agencies for investigation and prosecution of non-tax criminal laws.


There is also a broad quid pro quo clause under which the IRS can share tax return info with 3rd parties...

IRC 6103(k)(6) allows the IRS to make limited disclosures of return information in the course of official tax administration investigations to third parties if necessary to obtain information that is not otherwise reasonably available.

Which could allow IRS to share info with FBI or Grand Jury / US Attorneys / State AG's etc

NY law states
N.Y. TAX LAW § 697(e) (McKinney Supp. 1978-1979). Subdivision (e) also does not prohibit delivery of a copy of the income tax return to the taxpayer himself
or to the attorney general when a tax-related court action has been instituted. Id.
In addition, subdivision (f)permits disclosure of relevant tax return information to the federal government or other state governments whose taxing authorities allow reciprocal exchange privileges. N.Y. TAx LAW § 697(f)


So it appears that there are several back channel opportunities for the returns to have been shared without having to follow the traditional court order/subpoena process.....