MEXICO CITY (Reuters) - U.S. Border Patrol agents have come under fire twice along the Rio Grande in Texas in recent days amid rising tension on the frontier with Mexico, although no one was reported wounded, U.S. authorities said on Thursday. A Border Patrol spokesman said unknown gunmen fired on agents on patrol in Brownsville, Texas, late on Wednesday. It was not immediately clear if the shots came from Mexico or from within the United States. "Shots were fired, no one was injured and the FBI have taken the case over," Jose Rodriguez, a spokesman for the Border Patrol in McAllen, Texas, said by telephone. Rodriguez said the shooting was the second along the same stretch of the Rio Grande in the past week, after agents patrolling the area in a launch on Friday came under a volley of gunfire from Mexico. "On that occasion the shooters were hiding in brush on the Mexican side of the river ... The launch was struck by five bullets, although there were no injuries," he said. That incident came on the same day a Border Patrol agent fatally shot a teenage Mexican immigrant as he crossed the border near San Diego on December 30, triggering widespread anger in Mexico and calls for a full investigation. Speaking to Mexican diplomats late on Wednesday, President Vicente Fox reiterated calls by the Mexican government for clarification of the killing, and pledged to "ensure that total justice is done in the case." The 2,000-mile (3,200-km) U.S.-Mexico border has always been dangerous, although violent attacks on Border Patrol agents have risen in recent months, especially in Arizona, where around half the 1.2 million undocumented immigrants nabbed crossing from Mexico were detained last year. The Tucson sector Border Patrol said attacks on agents havroe almost doubled in recent months, and included cases in which officers have been shot at, rammed with cars and pelted with rocks by immigrants and smugglers.