The captured Nicaraguan "military man" who Secretary of State Alexander M. Haig Jr. told Congress yesterday was proof of Nicaraguan intervention here was identified today as either a newly trained future guerrilla platoon leader or an innocent student trying to go home.

Whatever he is, he is no longer in Salvadoran custody but in the Mexican Embassy, where he has asked for political asylum.

Until Monday, National Police Chief Col. Carlos Reynaldo Lopez Nuila said at a press conference, the police were questioning a young man who had been arrested Feb. 20 or 21 as he was coming through customs at Chinamas on the Guatemalan border. He confessed to having gone through guerrilla training in Mexico, Lopez Nuila said, and was being taken on Monday to the Mexican Embassy to spot a collaborator there when he "was grabbed" by people inside the embassy.

Other Salvadoran officials said interrogation revealed that the Nicaraguan clearly was a ranking official of the ruling Nicaraguan Sandinista National Liberation Front, sent to lead Salvadoran guerrillas.

In a statement from Mexico City, the Mexican government denied that version. Undersecretary of Foreign Affairs Alfonso Rosenzweig Diaz said the Nicaraguan is a student at the Autonomous University of Nuevo Leon in Monterrey, Mexico, who was traveling home by road during a semester break. Rosenzweig, and later statements by the Foreign Ministry spokesman, said the student had been "detained at the border, taken to San Salvador and interrogated. He then convinced the police that he had a 'contact man' in the Mexican Embassy" whom he was willing to turn in.

Taken to the embassy on March 1 by two security officials, the Mexicans said, the Nicaraguan, who was identified in all versions of the story as named Ligdamis Anaxis Gutierrez, broke away from them, banged on the door and told Mexican Charge d'Affaires Carlos Eduardo Amezquita that he was a Nicaraguan seeking asylum. Amezquita then barred the door to the pursuing Salvadorans, the Mexicans said. Gutierrez, the Mexican spokesman said, "was not pulled in the door by anybody."

The Mexicans said they had verified Gutierrez's student status with the rector of the Monterrey university and had, on the same day the Nicaraguan arrived at the embassy, informed the Salvadoran Foreign Ministry of his presence and requested that he be permitted to leave under diplomatic protection.

Mexico, which has advocated a negotiated solution to the Salvadoran conflict, has strained relations with the government here and maintains only a skeleton staff of three embassy officials. The Mexican Foreign Ministry said that two Salvadorans are also currently in the embassy, seeking asylum.

What has emerged as a somewhat confusing situation began yesterday, when Haig told a congressional subcommittee that the Salvadoran government had "today for the first time" captured "a Nicaraguan military man . . . sent down by the" Sandinistas to help run the military operation of insurgent guerrillas who are trying to overthrow the military-backed junta here. He said this was "unchallengeable evidence" and part of the "overwhelming and irrefutable proof" the U.S. government has to back its case for increasing aid to the Salvadorans.

Yesterday evening, however, CBS News quoted Salvadoran Defense Minister Jose Guillermo Garcia as saying that the Nicaraguan cited by Haig had been "snatched away" when he was taken to the Mexican Embassy to identify a suspected embassy contact with the Salvadoran rebels. Garcia said the Nicaraguan had been captured "three or four days ago."

Asked about the confusion today, State Department spokesman Alan Romberg said in Washington that he had "nothing further than what the secretary said yesterday" to add to the situation, and that Haig's comments "were based on a communication from our embassy . . . , which was relaying information from the Salvadoran government."

Asked later about the Mexican version of events, Romberg said, "It is at variance with the information we have received from the El Salvador government."

At the same time, State Department sources said a report about the Nicaraguan apparently had reached Washington from the U.S. Embassy in San Salvador on Wednesday, but had not been focused on until yesterday morning. The sources said they believed that the original cable had mentioned that the Nicaraguan already had escaped to the Mexican Embassy, but this apparently had been overlooked by Haig when he testified.

In Managua, the Nicaraguan government made no comment on Haig's charges.

Although the Salvadoran government declined comment on the situation early today, the version presented at the later press conference said that the man in question, a native of Managua, was traveling on a provisional Nicaraguan passport. He was stopped "when he couldn't specify in any exact manner what the purpose of his visit to El Salvador was, if he had any friends or where he would stay," Lopez Nuila said.

Under questioning, Gutierrez said he had fought the late Nicaraguan dictator Anastasio Somoza, joined a Sandinista student association and was sent to Mexico in October for four months of guerrilla training, Lopez Nuila related. Gutierrez reportedly described his training at the Institute of Coahuila in Saltillo.

At his center, Gutierrez allegedly said, there had been 40 trainees, half of them Salvadorans and half Nicaraguans. The director, who went by the name of Jorge, is "chief of urban resistance" in El Salvador and works in the Mexican Embassy with the ambassador under false Mexican papers, Lopez Nuila reported Gutierrez as saying.

Under interrogation, Gutierrez said he was told to return "to train combatants and to take command of a guerrilla platoon for a final offensive to begin in March," Lopez Nuila said. To contact Jorge, he was to come to the embassy wearing a green knapsack.

"We took him there with his green knapsack," Lopez Nuila related. Some cars were coming out as Gutierrez stood there with two police escorts, he said, and "some person took him by the shoulder and he was grabbed inside." The agents tried to get in but were told to leave because it was Mexican territory, Lopez Nuila said. "The next day the Mexican Embassy presented a request for political asylum for him and two others," men crippled by wounds. Those two, Lopez Nuila said, are "presumably terrorist combatants."

The police chief said four other Nicaraguans were arrested last week and another one killed after they assaulted and tried to extort money from a city resident. They confessed they were members of terrorist cell, Lopez Nuila said.

A U.S. Embassy official said "one would assume" that Gutierrez is the military man to whom Haig was referring, even though Gutierrez was captured last month and not last Thursday. "The Salvadoran government has kept us apprised of the situation from the beginning, at the very highest levels," the official said.

But Lopez Nuila said the first contact he had had with the embassy--the day State Department sources said they received the embassy cable--was Wednesday, when someone there called him and he sent the papers over. "They did not find out abou it from us," meaning from the police, Lopez Nuila said.