The border town of Brownsville, Tex., is at the front in America's "war" on drugs. Some authoritative sources say it is also a sieve for cocaine and marijuana.

Customs employees who have observed the Brownsville scene for years have tried in vain to alert the U.S. Customs Service that its Brownsville operations are flawed and possibly corrupt.

Last year the Drug Enforcement Administration warned Customs that it had received information that an average of three-fourths of a ton of cocaine a day was crossing into the United States at Brownsville, in produce, equipment or cattle trucks. But Customs insists it doesn't have an employee problem in Brownsville.

Customs sources, and internal documents obtained by our associate Dean Boyd, point a finger at the Brownsville Customs office of inspection.

"In a nutshell, the situation is completely out of control down there. Not only are they letting drugs across, but all sorts of other contraband," said Mike Busby, a former Customs inspector at the Brownsville Port.

We reported last month on the case of two Corpus Christi, Tex., Customs agents who claimed they found evidence of corruption in Customs while they were investigating a major drug ring. The two claimed that once they began to press for an internal investigation, they were harassed and threatened by their superiors. One of the agents was vindicated in a federal personnel grievance.

Additional documents we have seen echo the claims of those two agents.

In a September 1990 memo to the inspector general's office of the Treasury Department, which oversees Customs, one employee complained that Brownsville border employees were getting cozy with the wrong people.

Last April, in a report to Southwest Regional Customs Commissioner James C. Piatt, the same whistle-blower documented some ties between Customs employees and drug smugglers, and said some of the information was obtained from personal knowledge and "informants in Mexico."

In an affidavit to the Customs office of internal affairs in McAllen, Tex., former inspector Busby said he also had seen Brownsville employees let contraband into the United States.

Another Customs agent in Texas told us: "We spend so much time covering for ourselves, we don't have time to fight any war on drugs. You can imagine how effective we are."

Although the Brownsville Port has seen some management changes in the past year, Busby said "many of the same gangs continue to work down there." Another former Customs agent told us the changes were "largely cosmetic."

George Flores became the chief Brownsville Port inspector nearly a year ago after his predecessor was indicted, and later acquitted, on charges of smuggling liquor and saddles across the border. Flores says the latest allegations about the Brownsville operation "sound far-fetched. We have a good group of people here and we're tired of being beat up with these accusations."

A regional Customs spokeswoman said, "Based on the information we have been given, we have not been able to substantiate these allegations."