according to special relativity the mass of an object increases with its velocity by the lorentz factor. the lorentz faktor would be 2.3 for when travelling at 90% the speed of light. as you go faster and faster the lorentz factor increases. it would be 22.4 when travelling at 99.9% the speed of light(relative to our inertial frame of reference). that means that if something is travelling at 99.9% the speed of light it´s mass would be 22.4 times greater. in particle accelerators single particles get so fast they have a mass several thousand times greater than that same particle would have standing still. if, from that point on, you go on accelerating that particle even more, wouldn´t that particle at some point have gained enough mass that it turns into a black hole?