It certainly is admissible, and it does get shared with other agencies.
> Under §6103(i)(1), an assistant U.S. attorney may obtain tax returns as part of a non-tax criminal investigation or grand jury proceeding by submitting an ex parte application to a federal district judge. Taxpayers have no right to notice, a hearing, or dis-closure of the application,1 and prosecutors may file simultaneous motions to seal both the application and subsequent order granting or denying the application.2 The district judge “may” grant the order if (1) there is reasonable cause to believe a criminal act has been committed, (2) there is reasonable cause to believe the tax return is relevant to the commission of the criminal act, and (3) the return is sought exclusively for use in a federal criminal investigation or proceeding and the information sought cannot reasonably be obtained from another source.
> In preparing for trial, defense counsel in both tax and non-tax cases need to anticipate that the government might attempt to offer tax returns not directly at issue in the case to show knowledge and intent, unexplained wealth, or even the falsity of rep-resentations made in connection with a fraudulent scheme. While some courts have permitted the gov-ernment to introduce tax returns as circumstantial evidence of some disputed fact...
> Under §6103(i)(1), an assistant U.S. attorney may obtain tax returns as part of a non-tax criminal investigation or grand jury proceeding by submitting an ex parte application to a federal district judge. Taxpayers have no right to notice, a hearing, or dis-closure of the application,1 and prosecutors may file simultaneous motions to seal both the application and subsequent order granting or denying the application.2 The district judge “may” grant the order if (1) there is reasonable cause to believe a criminal act has been committed, (2) there is reasonable cause to believe the tax return is relevant to the commission of the criminal act, and (3) the return is sought exclusively for use in a federal criminal investigation or proceeding and the information sought cannot reasonably be obtained from another source.
> In preparing for trial, defense counsel in both tax and non-tax cases need to anticipate that the government might attempt to offer tax returns not directly at issue in the case to show knowledge and intent, unexplained wealth, or even the falsity of rep-resentations made in connection with a fraudulent scheme. While some courts have permitted the gov-ernment to introduce tax returns as circumstantial evidence of some disputed fact...
https://www.maglaw.com/publications/articles/2015-05-21-the-...