Complex sentence: Their IFF systems responded negatively to the attempt to identify the contact on Mode I. The Mode IV momentarily gave a positive response, but thereafter responded negatively and the F-15s moved to intercept the unidentified aircraft.
Split into simpler sentences: Their IFF systems responded negatively to the attempt to identify the contact on the first frequency. The second frequency momentarily gave a positive response, but thereafter responded negatively and the F-15s moved to intercept the unidentified aircraft.