The idea that the IRS considers political speech by nonprofits to be sufficiently illegal to justify sending information to the FBI is a blatant violation of the First Amendment. If there are issues with an organizations' tax status, the IRS is the appropriate agency to deal with that. But the Internal Revenue Service admitted the law did not allow them to do anything, because under the existing laws, regulations, and Supreme Court precedent, what the groups being targeted were doing was legal.
So they sent their database of targeted organizations to the FBI, months before the election, in the hopes that the FBI could find something to charge. The database contained, by the IRS's own admission, non-public confidential taxpayer information. That's a felony, and likely why Lois Lerner pled the 5th.
The documents were subpoenaed as a part of the Oversight Committees ongoing investigation into the IRS targeting of right-leaning groups, which took place against the backdrop of the Citizens United ruling. E-mails cited in a committee report released in March show that the decision caused a lot of angst for Lerner and her colleagues in the IRSs Exempt Organizations division, and she noted in public remarks that the agency was under pressure to fix the problem created by the decision.
One more thing to consider. We have evidence now that the IRS, FBI, FEC, BATFE, OSHA, and other agencies were collaborating to persecute political opponents of this administration. Who could instruct those agencies to work together on a project like this?