No one every considers the poor psion.
There's no better detective/inquisitive in 3.5e than a clairsentient psion. If anyone on the Eberron design team had even a scrap of interest or familiarity with psionics, they wouldn't have wasted the space on the prestige class.
Take a level of rogue to give yourself some combat utility. A level of fighter could also be hand here, since manifesting psionic powers is not affected by wearing armor. Spend the other four levels on psion. Take the following powers as a start:
1st: Empathy, Telempathic Projection, Disable, Precognition, Inertial Armor (or Force Screen)
2nd: Object Reading, Sensitivity to Psychic Impressions, Cloud Mind, Psionic Tongues
The two key powers are Object Reading and Sensitivity to Psychic Impressions. It can speed up murder investigations, since any object used to attack someone will pick up the user's "impression". The second is also handy for murder investigations, but any emotionally charged events, like a heated argument, also leave their mark. Motives become easier to discern. Couple these two powers with the fact that many psionic powers can be manifested with no display with a Concentration check means that the psionic investigator can use his powers in situations that an arcane caster can't without attracting too much attention.