Workload; Distraction; Confusion
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Gate; General Maintenance Action; General Declared Emergency
Aircraft
Single engine taxi out to Runway XXL for departure; 40 seconds after #1 engine start rollback; we got a 'Source Off LT and Gen Offline LT.'  20 seconds after that; Tower notifies us we have a fire in one of our engines. Simultaneously; a passenger on the Captain's side tells our flight attendants 'he sees a fire on the #1 engine'; and the flight attendants are passing that info to us in the cockpit via 4 dings and through the headphone.  By now; we also have an 'ENG FAIL' warning for #1.  Max EGT seen on #1 was 450.  EGT at the ENG Fail was approximately 350.  No fire or OVHT light appeared; however the Tower and another aircraft were still screaming that we were on fire.  We executed the Engine Fire Emergency procedures for the #1 engine; established communication with the Fire Chief; who stated that the fire looked to be from the rear of the engine only and he saw no visible damage to the aircraft.  Normal shutdown and completion of checklist items for the emergency.  All checklist items completed and uneventful exit from the aircraft at the gate.  Maintenance was already on hand and found a few initial assessments via their BIT examination.  During the initial fire; our B Flight Attendant and a few passengers also stated they heard a bang and saw several long flames come from the back of the engine.  Captain; First Officer and flight attendants all debriefed each other; and we completed the rest of our day with three more legs.
[Narrative #2 contains no additional information.]
A B737-700 Left Engine shutdown about 40 seconds after start and passengers began reporting a fire so the crew shut the engine down with Engine Fire Checklist while the airport fire crew stood by. At the gate Maintenance determined was a tail pipe fire because of a fuel control issue.