The first bullets struck El Paso's city hall at the end of a work day. The next ones hit a university building and closed a major highway.

Shootouts in the drug war along the U.S.-Mexico border are sending bullets whizzing across the Rio Grande into one of the nation's safest cities, where authorities worry it's only a matter of time before someone gets hurt or killed.

At least eight bullets have been fired into El Paso in the last few weeks from the rising violence in Ciudad Juarez, Mexico, one of the world's most dangerous places. And all American police can do is shrug because they cannot legally intervene in a war in another country. The best they can do is warn people to stay inside.

"There's really not a lot you can do right now," El Paso County Sheriff Richard Wiles said. "Those gun battles are breaking out everywhere, and some are breaking out right along the border."

Police say the rounds were not intentionally fired into the U.S. But wildly aimed gunfire has become common in Juarez, a sprawling city of shanty neighborhoods that once boomed with manufacturing plants. It's ground zero in Mexico's relentless drug war.

More than 6,000 people have been killed there since 2008, when the Sinaloa and Juarez cartels started battling each other and Mexican authorities for control of the city and smuggling routes into the U.S. Nationwide, more than 28,000 people have been killed since President Felipe Calderon launched his offensive against the cartels shortly after taking office in December 2006.

Until now, communities on the U.S. side of the border have been largely shielded from the violence raging just across the river. But the recent incidents are the first time that live ammunition has landed in American territory.

On Saturday, as gunmen and Mexican authorities exchanged gunfire in Juarez, police in El Paso shut down several miles of border highway. Border Patrol spokesman Doug Mosier said his agency asked for the closure — a first since the drug war erupted — "in the interest of public safety."

No one was injured on the U.S. side, but one bullet came across the Rio Grande, crashed through a window and lodged in an office door frame at the University of Texas at El Paso. Police are also investigating reports that another errant round shattered a window in a passing car. Witnesses at a nearby charity said at least one bullet hit their building, too.

El Paso police spokesman Darrel Petry said authorities have only confirmed the single bullet found at the university, but it is possible that several other shots flew across the border.

"As a local municipality, we are doing everything we can," Petry said. "Looking where we're at, the community we live in, that's all we've got. It's the reality of life here in El Paso for right now."

Officers say the types of bullets used in the drug war can travel more than a mile before falling to the ground.

In Saturday's shooting, the bullet that hit the campus building may have flown just under a mile before lodging in a door jam. Back in June, at least seven shots fired from Juarez flew more than half a mile before hitting City Hall.

In some places, El Paso is separated from Juarez by little more than a few yards of riverbed.

Andrew Kunert was napping Saturday when police started banging on his door at an apartment building just feet from the border. He said officers with high-powered rifles slung across their chests warned him to stay inside and away from windows until the shooting stopped.

The rat-a-tat-tat of gunfire to the south is nothing new, but bullets coming north is a worrisome new development, Kunert said.

"About once a week, you can hear gunfire," he said. He worries about the children who live at the Old Fort Bliss apartment building and routinely play outside when gunmen are trading shots across the river.

At the Rescue Mission of El Paso, kitchen manager Bill Cox said several bullets hit a pair of old silos on the charity's property, which is down a hillside from the university campus. Volunteers and homeless people coming to the mission for food or other help could easily be in the line of fire, he said.

"Someone can be walking down the street out here and be hit," Cox said.

In a letter to President Barack Obama after the City Hall shooting, Texas Attorney General Greg Abbott said it was "good fortune" that no one was injured and insisted the shooting was evidence of the need for more border security.

"Luck and good fortune are not effective border enforcement policies," Abbott wrote. "The shocking reality of cross-border gunfire proves the cold reality: American lives are at risk."

And Monday, Texas Gov. Rick Perry issued a statement demanding more security.

"It's time for Washington to stop the rhetoric and immediately deploy a significant force of personnel and resources to the border to protect our homeland," Perry said.

Katherine Cesinger, a Perry spokeswoman, said the governor believes that more security — in the form of federal agents and even troops — could all but shut down the border to smuggling and help put Mexico's warring cartels out of business.

The only way cartels "are being successful is by being able to operate on both sides of the border," Cesinger said. "If you shut down that border, they are out of business. They are not able to continue."

Obama has ordered about 1,200 National Guard troops to the border in California, Arizona, New Mexico and Texas to help the Border Patrol and officials from Customs and Border Protection.

But the federal government has insisted that the troops will only help federal agents with intelligence, surveillance and other duties that do not involve actually arresting anyone.

Sheriff Wiles says more security in El Paso won't solve the problem because the war is in another country.

"Juarez is experiencing a major wave of violence, and we are feeling some of that," Wiles said. "I don't know of any way around that. Until that issue is resolved in Juarez, we are going to be dealing these kinds of things."