After several unsuccessful start attempts in a CRJ-700; fuel fumes were reported in the cabin. After gate return fuel leak reported on ramp.

Narrative

Operating flight from ZZZ to ZZZ1 a potential exposure to noxious fumes occurred. In addition; a substantial delay was incurred because of a related mechanical issue. Upon push-back; first flight on the day; engine 1 was started. Fuel valve test was completed. Upon engine 2 start; the engine failed to light. N2 was approximately 50%; Interstate Turbine Temperature (ITT) was normal; N1 failed to indicate any movement. After the 90 second limitation was reached; the engine start was aborted. After a cool-down period; a second engine start attempt was made. After 3 sequential engine starts to no success; the forward Flight Attendant called the flight deck and notified the flight deck that a very strong odor of jet fuel existed inside the passenger cabin. After receiving the notification from the flight attendants; the aircraft was taxied back into the gate area under own power with engine number 1. Passengers were then asked to deplane and gather their belongings. After speaking with Maintenance Control and having contract Maintenance inspect the aircraft; no solution was found. Also noteworthy; no persons on board the aircraft exhibited adverse effects from the fumes. Ground control notified the flight crew via radio on ground frequency that the right (number 2) engine had fuel leaking from the engine cowling. Simultaneously; a flight crew heard the conversation on frequency and said they also could smell fuel in their aircraft supposedly coming from our plane. Airport Fire and Rescue was dispatched to contain any potential fire and to also clean up the fuel that was expelled onto the ramp area. Unknown mechanical failure.