South Texas sheriff's deputies were investigating Thursday whether Mexican gunmen who fired on deputies and Border Patrol agents from across the Rio Grande had crossed into the U.S.

Hidalgo County Sheriff Lupe Trevino said 200 to 300 shots were fired from automatic weapons Wednesday night, but no one was injured on the U.S. side and police did not fire back.

"This type of incident is a very good example of why I will not allow my deputies to patrol the river banks or the levees anywhere close to the river," he said. "We do have drug trafficking gangs, human trafficking gangs, that will not hesitate to fire at us."

Trevino said the shooting appeared to have started in Mexico, at a riverside ranch owned by a family from Donna. Two brothers said they were with their father at the ranch when vehicles full of armed men drove into the ranch and opened fire on the ranch house, killing a ranch hand and taking their father hostage, Trevino said.

The brothers hid for several hours in their corn field before swimming across the river, which is at that point only about 30 to 40 yards wide. They called their mother from a cell phone, who called police.

The mother said someone may have been killed, and police and the Border Patrol initially went to the river bank to search for a body. Once there, the gunfire began.

"There is no doubt about one thing, that we were shot at from the Mexican side," Trevino said, "a barrage that lasted over five minutes, maybe even seven."

He said the deputies did not shoot back because they could not see the assailants through the trees on the other side.

On Thursday, a police commando team finished securing the area and deputies were to begin searching for bullet casings that would prove some shots were fired from the U.S. side.

Trevino said such proof could lead to charges of attempted capital murder, though he said the chances of finding and prosecuting the gunmen were "next to impossible."

Trevino said Mexican police are conducting their own investigation but had not yet been in contact.

He said it was too early to determine a motive, but theories were that a drug gang was trying to get control of a ranch adjacent to the river or that gang members thought there were drugs on the ranch to steal.

U.S. Customs and Border Protection statistics indicate violence on the border has escalated.

In the Rio Grande Valley sector alone, there have so far been 76 reports of violence against Border Patrol agents since the start of the fiscal year Oct. 1, including shootings, physical assaults, vehicle assaults, threats, and rock throwings. There were 35 such reports for the previous fiscal year.

Border-wide, there were 566 assaults against agents for fiscal year 2005, compared to 548 in 2004 and 375 in 2003.

Border Patrol spokesman Roy Cervantes said most of the violence was a result of increasing enforcement which is making smugglers more desperate.

"Front-line agents in the Rio Grande Valley sector know full well what impact their efforts are making on these criminal organizations," he said. "Unfortunately these organizations' frustrations are becoming more violent."