Severance Agreement accepted on both sides, employee signs, can the employer rescind?

I worked for an organization for more than 24 years. I progressed through various roles, increasing responsibilities, and was considered the top performer in my executive position. My child was experiencing a critical medical issue, which began in October of 2014. The condition progressed and became life threatening in March 2015 and I requested FMLA to provide care. Upon attempting to return (with a limitation expectation from my childs health care provider) I was offered a limited temporary position (after which I must return to original position without limitations) or a severance package. The decision was excruciating, as this was my lifes work, however, after discussion with my attorney we negotiated minor changes which were agreed upon and accepted. I immediately signed, with notary public signatures as well and 3 original copies were sent to the employer. The 7 day "change my mind" time frame passed and I still received no word. I asked about returning property and retrieving mine - which was met with no communication at all. My lawyer pressed for answers and was told that there was now an investigation into a potential policy violation prior to my FMLA (more than 3 months ago - at the peak of my child's condition)...to which they indicated the offer may be rescinded. Is this legal?