Air carrier Captain reported fumes in the cockpit during deplaning. All passengers and flight crew were evacuated and Airport Rescue & Fire Fighting was summoned.

Narrative

Approximately halfway through deplaning Aircraft X from ZZZ1 to ZZZ; crew rapidly noticed fumes in the cockpit. Crew was alerted immediately. Captain encouraged Flight Attendant (FA) to continue the deplaning process and get the remainder of the pax (Passengers) off of the plane. Captain also immediately powered down FMSs; and avionics; and checked all voltages and battery temps. Crew attempted to determine cause and location of fumes. Pax were all off of the aircraft when crew then noticed smoke coming from the left hand distribution box behind the Captain seat. At that point; Captain told crew to deplane; switched to GPU power briefly; alerted operations and requested fire response; and then completely powered down the aircraft. Fire response was quick. The entire plane and outside of the aircraft smelled of burning plastic; and the smoke at that point was dissipating and improving. Captain alerted Maintenance; Dispatch; chief pilots; and followed up with crew to make sure everyone was okay.Cause: Possibly an incorrect screw used by Maintenance to secure an electrical component in distribution box.