A fire broke out at a state prison in the northern Mexico town of Monterrey Tuesday morning, killing at least 20 people and injuring an undetermined number, Mexican television reported. Mexico City-based Channel 2 television reported that the fire broke out at 2:15 a.m. at the state prison in the northern industrial town about 425 miles north of Mexico City. The report did not say what caused the fire. The prison fire occurred two days after an explosion at a fireworks shop ripped through makeshift stalls and buildings near a Mexico City market crowded with Christmas shoppers, killing at least 62 people and injuring 100.