THE Mexican government has acted swiftly to get to the bottom of an incident last month that appears to implicate elements of the Mexican Army in drug trafficking. Now, with many of the facts known, the official investigation and prosecution branches of the government and the military must bring the matter to a conclusion that will wash in both Mexico City and Washington.On Nov. 7 a plane carrying nine Mexican drug-enforcement agents landed on a rural airstrip near Veracruz, in hot pursuit of a plane carrying cocaine. The agents were engaged by an Army unit at the airfield, and in a two-hour firefight seven agents were killed. The fighting continued even though an Army general on the scene was notified by phone by police authorities that the soldiers were firing on drug agents, and even though the agents tried repeatedly to identify themselves. The three crew members of the coke-carrying plane escaped. The suspicious facts were recently disclosed in a report of the National Human Rights Commission, established last year by President Carlos Salinas de Gortari. The commanding general and several other military officers have been relieved pending review of the commission's findings. The case raises troublesome issues about corruption in the Mexican military and, particularly, about the extent of military involvement in aiding or protecting drug traffickers. And it is embarrassing for President Salinas, coming on the eve of a trip to Washington this month. But the significance of the episode shouldn't be overstated. The Mexican military is heavily committed in the war against drug traffickers. That very commitment creates occasions for temptation in the front lines. But Mexican military authorities are as eager as their civilian counterparts to keep the Army untainted by drug corruption. The government's handling of the case thus far is commendable. The episode needn't be more than a cloud's shadow on the strong and constructive relationship that Mexico and the US have forged.