Police this afternoon arrested Raul Salinas de Gortari, brother of former Mexican President Carlos Salinas de Gortari, in connection with the assassination last September of the ruling PRI party's No. 2 official. The stunning twist in one of the country's several unsolved political crimes increased speculation that two potential Mexican presidents -- one a candidate, and one a popular reformer -- had been killed in a clandestine party feud. It was unclear whether Raul Salinas would be charged with the murder of the PRI official, Jose Francisco Ruiz Massieu; the Mexican attorney general's office scheduled an evening news conference to discuss the case. (Investigators recently had arrested a second suspected gunman in the other major killing -- the March, 1994, killing of PRI's presidential candidate Luis Donaldo Colosio.) Earlier today, Ruiz Massieu's own brother, former deputy attorney general Mario Ruiz Massieu, demanded tougher government action to solve a crime he blamed on high-level PRI figures.