Federal agents say that just southeast of here, tons of marijuana, protected by Mexico's Federal Judicial Police and the local Chihuahua state police, regularly roll into the Mexican town of La Isla, a dusty collection of small houses built of scrap metal and adobe.

Based just across the Rio Grande River from Fabens, Tex., the drug traffickers rely on the incentive of escaping poverty to provide a small army of backpackers, eager to earn up to $200 a trip by strapping 100 pounds of marijuana to their backs and wading across the river to "stash houses" on the U.S. side. A 4,400-pound load of marijuana was seized on the northern side of the border earlier this month.

To the east and the west, federal law enforcement officials say, the picture is much the same. Marijuana in record quantities is streaming across the border on foot and in four-wheel drive vehicles. Overhead, low-flying planes carry cocaine and heroin, landing at private airstrips, on straight stretches of highway or even on the desert floor.

U.S. Customs Commissioner William von Raab, calling the southwest border a "modern-day horror story," said he has privately asked the Reagan administration to declare it a "crisis zone."

In making the request, he cited runaway drug trafficking, violent attacks on federal agents, suspicious cash movements in the area, and ongoing problems with Mexican authorities.

In addition, von Raab has charged that Colombian drug traffickers have moved in along the southwest border -- on both the U.S. and the Mexican sides -- because it is now easier to smuggle drugs across that border than into Florida. He said the Colombian traffickers are paying Mexicans to provide protection and transport the drugs.

Sources at the federal Drug Enforcement Administration confirm that they have seen a sharp increase in Colombian involvement along the Mexican border in about the past 10 months.

"It's obvious that it's easier [for the traffickers] than Florida," von Raab said in an interview earlier this month.

"If a bad guy comes into Florida, we can chase him," he said. "But with the Mexican situation, all they have to do is step three feet over the border and thumb their noses at us -- and they do . . . . The Rio Grande is the line behind which the bad guys can take refuge."

Federal officials say that as drug enforcement resources were redirected into southern Florida, smugglers began to realize that the southwest border with Mexico was left unguarded.

Horace Cavitt of the El Paso Intelligence Center, a multi-agency organization that concentrates on smugglers of narcotics, firearms and aliens, said, "It's easier to get the dope in here. Police are running all over each other in Florida and the Mexican border has been ignored. It's 2,000 miles long, and there are a million places you can cross."

Saying that the traffic is "escalating rapidly," Cavitt added, "We're knee-deep in cocaine. Seizures are up, and prices are down."

Von Raab said drug enforcement along the Mexican border has deteriorated badly since the late 1970s when Mexican authorities actively assisted U.S. agents.

He said it is "10 times worse than three or four years ago. It's a runaway operation right now . . . . It's a no-man's land."

He added that the situation is worsening as the Mexican economy deteriorates.

In his efforts to persuade the administration to redirect resources toward the southwest border, von Raab has cited several developments that he finds alarming:

*Glenn Miles, a Customs patrol officer, was murdered Feb. 21 near Sells, Ariz., by three drug smugglers, believed to be working for Colombian traffickers, who fled back across the border into Mexico.

*During a three-day period early this month, more than 4,200 pounds of cocaine was seized on both sides of the border from a trafficking group bringing the drug through Tijuana, Mexico, into the southern California area.

*There have been about a half-dozen firearms attacks on Border Patrol agents and stations in the past several months.

*Major changes in financial patterns are occurring along the southwest border. For example, von Raab said, the Chamber of Commerce for the Lower Rio Grande Valley, one of the poorest areas in the nation, has reported that, as of January 1985, deposits in local banks showed an increase of $720 million over the same period a year earlier.

The Federal Reserve Bank in El Paso, which showed a negative cash flow in 1981 and 1982 as dollars flowed away to other parts of the country, reported a $600 million cash surplus for 1984.

Barriers to smugglers attempting to cross the border are virtually nonexistent. Starting in El Paso and extending eastward, the Rio Grande is shallow enough in many areas to wade across. In the places where there are chain-link fences, there are gaping holes.

West of here, the "border" consists mostly of a waist-high, four-strand barbed wire fence. In many spots it has been cut down or simply knocked down by four-wheel-drive vehicles.

Customs agents freely admit that they are understaffed along the border and that there are stretches of as much as 150 miles between border stations.

James Wait, the Customs agent who heads enforcement for the El Paso region, adds that planes carrying drugs regularly arrive undetected from Mexico because the air radar coverage along the southwest border is the worst in the country.

Many of the planes fly low through the valleys of mountain ranges that extend south to north -- flying without lights and with their identification numbers taped over. On the rare occasions when they are detected, they make a dash for the border. "They know we can't follow them," said Al Souza, a Customs agent based in Tucson.

Wait cited Customs charts of the border showing that in many areas military radar coverage starts only at 14,000 feet and above. The radar is intended to detect incoming missiles, not small drug planes.

One agent added, "I don't know what they think we're going to do if the Russians decide to fly in over Mexico at 5,000 feet."

Von Raab has been scathing in his criticism of Mexican authorities' lack of cooperation.

"Mexico is . . . a serious national security concern," he said. "If law enforcement can be so easily bought by drug smugglers, it can be just as easily bought by terrorists."

Leonardo French, a spokesman for the Mexican Embassy, has responded, "Mexico has done its best for many years and is firmly determined to continue to do so in the war against drug trafficking. But in order to have any possibility of success, it is indispensable to attack simultaneously all the links in the chain of this criminal activity . . . as well as to discourage or even penalize demand and consumption. Most of these links do not have their main bases in Mexico.

"Sometimes we think it is unfair to keep on criticizing the actions of Mexican authorities . . . . Instead of strengthening the necessary cooperation between our nations, the criticism favors the drug traffickers who certainly take advantage of it."

Von Raab was especially critical of a recent incident in Naco, Ariz., in which an agent opened the trunk of a car and found 150 pounds of marijuana. The Mexican driver assaulted the Customs inspector, who grabbed him by the jacket. The man escaped, leaving behind the jacket identifying him as a Mexican Customs officer.

"We passed the word that we were very unhappy," the customs chief said. "They first denied he was Mexican Customs. Finally, they said they would deal with him administratively. I was in Mexico recently and asked for a status report. All they did was ask for his jacket back." Von Raab returned the jacket.

"Their typical response is, 'Oh, we're very concerned. Give us the specifics.' But unless you can provide the name, rank and serial number, the whole thing is dropped," von Raab said.

He added that the Mexican government has refused to agree to proposals to allow Customs planes to chase smugglers into Mexico with Mexican authorities aboard.

Without more help from the Mexicans, he said, the border situation is almost hopeless.

"The first thing that has to be done is to get the cooperation of noncorrupt Mexican officials," von Raab said.

"As long as it's a safe haven," he said, "we can put thousands of people into Customs uniforms, but we can't do anything."