Using law enforcement to protect the people in aviation is nothing new. However, the NTSB wants the power to grant immunity during air crash investigations. One problem is that they have not worked in criminal law. We use our laws to weed out criminals from the medical profession and other areas, and the flying public needs to also be protected. You need legal proceedings and a person is called for questioning, and must refuse to cooperate, taking the 5th Amendment before you would even consider seeking immunity.
Someone needs to refuse to cooperate first, because they fear prosecution.
You must follow the federal, state, and local laws that apply. Everyone wants to be granted immunity, but keep in mind it was intended for use with criminal law enforcement, and by granting immunity a successful criminal might be free. The manufacturers have impeded adequate emergency evacuation standards. The aviation rule making is controlled in the U.S., and although Europe is trying to have more influence, we need your help in the U.S. to demand the highest standards. You have influence and clout, sadly that will grow partly because of the ongoing spate of disasters.