Light Transport
Flight Crew Instrument; Flight Crew Commercial; Flight Crew Multiengine
Flight Crew Last 90 Days 153; Flight Crew Total 1350; Flight Crew Type 154
Situational Awareness; Confusion
Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway; Ground Incursion Taxiway
General None Reported / Taken
Airport
I am a First Officer and was Pilot Monitoring so I was handling radios. We were issued a taxi clearance at IAG to take Taxiway D to Runway 28. When we approached the intersection at D1 we were confused as to if we should stop or proceed to D for the beginning of the runway. I'm unsure of the taxiway markings but during my duties and before I knew it we were in the threshold area for Runway 28.The Captain spoke with ATC and they said it happens all the time and the signage isn't very clear. The confusing taxiway and being on the back half of our trip and fatigue most likely contributed to the Captain's mistake taxiing. I think clearer taxi instructions or new signage is needed at IAG to prevent further mistakes and avoid any potential accidents from accidentally taxiing onto the threshold.
The weather was better than 10 miles and 4;000 scattered. Were cleared to taxi to Runway 28 via Delta and Delta 1. As we proceeded on taxi way D; we missed Delta 1 and continued to taxi on Delta and crossed the 28 ILS runway critical line. At this point we were notified by ATC that we crossed the approach end of 28R and when ready; we have a number for you to call for possible pilot deviation.Possible causes:Fatigue; this was day six of flying and we have had some long days during the week. This day began at XA:15 am and this event occurred at XK:50.Preventative measures - Fatigue mitigation recognize it before it onsets.  Better markings; a sign. We are having a briefing on this situation on our next safety meeting.
Flight crew reported a runway and taxiway incursion while taxiing to the runway at IAG airport. The crew cited signage issues contributed to the event.