I think FBI was hoping to interrogate him and get information or a confession from him. They didn't really have enough evidence at that point that would allow them any hope of a conviction based on evidence alone. When he got a lawyer they had to let him go, but they didn't stop investigating him. According to his own account they followed him for months; he even grew a beard because of it.
Basically, the lawyer wouldn't allow questioning of him and they (FBI, LE) were not ready to take the case to court.
That's my take.
Then, when Kevin's information came along LE second-guessed themselves and took the investigation in another direction. What I wonder is WHY they assumed the car Kevin was driving caused THE TRACKS that they had already connected to DJH's car?? It seems like a lot of assumptions were made... like "now we know Kevin was there, that must have been who DR saw pulling around in the driveway, oh boy the abductor must have been on foot". It really was not investigated very thoroughly at that point, imo.