F18 chasing an unidentified aircraft

I saw one F18 flying low over my neighborhood and watched as it sped up. I then noticed it was chasing something. I am unsure of the shape or what it was it the F18 was chasing, but it was fast and flew up quickly into the atmosphere while the F18 tore West and then directly up. I could no longer see either the F18 or the UFO, but continued to hear the F18 jet as it was still audible. I couldn’t hear the UFO at all. Luckily, another bystander witnessed this incident and we were both unable to articulate precisely what the F18 was chasing. Furthermore, I had a hard time understanding why just one F18 jet was chasing this, as from my understanding they typically do not travel alone.