“Do I need a lawyer if I am under investigation and no charges have been filed against me?”
Yes, your need for effective defense is just as great during the course of the police investigation. It is important to know that police and prosecutors usually do not give people the benefit of doubt before they file charges. They often make mistakes, and they are not always thorough. Don’t assume that the police will fairly decide whether you should be charged.
Any evidence in your favor needs to be investigated and preserved by your lawyer as early as possible. In some cases he is able to bring evidence to the attention of the police and prosecutors before charges are filed and persuade them not to file charges against you. It is often easier to convince police and prosecutors not to file charges in the first place than to persuade them to dismiss charges once filed.
It is often easier to preserve witness statements, documents, and and other evidence before a charge has been filed than it is to go back and get them afterward. And it is ALWAYS better to have the advice of a competent lawyer when the police or regulators are investigating so that you do not do or say anything that will cause you problems later.