Air carrier First Officer reported a fumes event of unknown source during preflight where the flight crew was experiencing physiological symptoms. The Captain refused the aircraft; the flight was cancelled; and maintenance action was taken.

Narrative

When the Captain and I boarded the plane the flight attendants let us know there was a burning electrical smell back by door 3L. The Captain spoke with the FAs and then wrote the issue up and called Maintenance. Maintenance personnel came to the aircraft and began investigating. At least one of them said they could smell the odor as well. They investigated but could not find a source. The Captain and I went back to the area and could no longer smell it at door 3L. However; there was a strong odor that irritated my nose and the Captain's eyes around rows 26/27. Maintenance again investigated and switched on and off the packs and recirculation fans. They were unable to determine a source and said the fumes had gone away and that would sign off the discrepancy. The crew was unsatisfied with the answer and the Captain initiated the process to reject the aircraft. While this was happening the Captain and I were sitting in the flight deck with packs off. I felt like I was getting a small headache and just needed some fresh air. I left the flight deck for a few minutes thinking maybe I was smelling something there. I came back a few minutes later and could definitely smell faint fumes. I asked the Captain to leave and come back and to see what he thought. He was also complaining of a slight headache that had started. He left and came back a few minutes later and could smell something as well. So we initiated a new maintenance write-up for the flight deck.New maintenance personnel came out and multiple members of the team were able to detect the smell as well. There was approximately 2 hours of time lapse between us first getting on board the aircraft and the flight deck odors. At this time the Captain had already rejected the aircraft but now had additional symptoms and a few members of the crew (FAs) were complaining of headaches as well. The Captain spoke to medical personnel who determined he was unfit to fly. However; my symptoms had subsided and I was deemed fit to fly. While this was going on the flight was officially cancelled. We were rescheduled to operate a ferry flight from ZZZ to ZZZ1 the next morning.