It apparently began with a letter Tom Posey sent to Honduran Gen. Gustavo Alvarez Martinez last November.

"We got the idea from the newspapers," Posey said. "He was the strongman."

A U.S. Marine veteran who had founded a group called "Civilian-Military Assistance" to help counter communism in Central America, Posey says he addressed the letter quite simply to "Gen. Martinez, Honduran Government, Honduras, Central America," and mentioned the group's hopes of doing something tangible to stem the tide.

"I was tickled pink when I got a letter back," Posey recalled. "It really inspired us. He invited us down. He said he was delighted to hear from people who were interested in doing something."

Their exchange led to a series of expeditions that ended tragically last weekend when two American members of CMA were shot down and killed in a rebel helicopter over Nicaraguan territory.

The Sandinista government of Nicaragua promptly charged that the Americans were "CIA mercenaries." U.S. officials just as promptly denied any connection with CMA, and Posey's group also says it was independent, operating without consorting with any U.S. government agency.

There is, however, evidence that U.S. agencies and officials did have knowledge of the CMA operation, and facilitated it along the way.

Posey had earlier been granted a license by the Treasury Department as a firearms dealer. He stated on the application, "I plan to buy weapons and ammo to send to El Salvador with that government's permission."

There are also indications that U.S. diplomatic and military personnel in Central America gave various kinds of help to Posey and his followers on their expeditions there.

"It's a lot easier than most people think," said Walton (Cisco) Blanton, another CMA founder and former member of an Alabama National Guard Special Forces unit, referring to the small group's moves to help first the government of El Salvador, then the rebels in Nicaragua over the last year. "All you've got to do really is reach out . . . . "

"And go for it," Posey said, finishing the thought.

The first trip to Honduras, for example, took place last January. Posey said he and three other CMA members went after notifying the Honduran Embassy in Washington of sidearms and rifles they intended to take with them, complete with serial numbers.

"We took weapons with us, yes sir, for our own protection," Posey told reporters here today. He said they also took about $4,000 worth of supplies, mostly medical equipment, and flew to Tegucigalpa on a private plane they had chartered.

"We'd just gotten a brand new bunch of uniforms" for the Nicaraguan rebels, Posey added. "We figured we could save $500" -- in airline tickets and freight costs -- "if we were to fly ourselves down there."

Landing in Honduras was no problem.

"When we got to Customs," Posey recalled, "we showed them the letter from Gen. Alvarez Martinez. The guy looked at the letter, closed it up real fast and passed us right through."

The men took a cab to a local hotel, across the street from security police headquarters, and, Posey says, proceeded to the U.S. Embassy first thing the next morning.

"We showed the letter to an American, he had a business suit on," Posey said. "I said, 'Hey, Bud' -- I called him 'Bud' because he was younger than me -- I said, 'We came here to check in.' "

He said the embassy official made a telephone call and told the group to go back to their hotel.

"We told them what we was wanting: to meet with an official of the Honduran government. I don't know who he called. They said go back to the hotel and they or somebody will contact you."

About three hours later, Posey said, a Honduran official came by. Once again, "we showed him the letter." Alvarez, who was later ousted, was reportedly out of the country at the time, but Posey said the CMA group spoke with an aide to the general and was eventually put in contact with rebel forces of the Nicaraguan Democratic Front.

"We went to a base camp that was told to us was in Honduras," Posey stated. He said they presented their supplies, but declined to say whether they also provided the rebels with basic military training, another service that CMA was set up to provide, on that first trip.

The January trip was CMA's first in support of the Nicaraguan rebels, but not the first south of the border. Posey, Blanton, and several others who had "known each other for years" decided in July 1983 to try to do something "as private citizens" and focused first on helping the government in El Salvador.

A produce dealer from Decatur, Ala., Posey said he scraped together some savings and made the trip to San Salvador on his own last October. He said he stretched his cash by taking a Greyhound bus to New Orleans for the Taca Airlines flight.

"I took some rations with me to make sure I had something to eat," he added with a grin.

His aim, Posey said, was "to find out what they in El Salvador needed," especially in light of congressional reluctance at the time to supply more funds.

Posey said he went first to the American Embassy as he did later in Honduras and encountered a U.S. military officer who was "coming in the door."

"I said, 'Hey Bud, I need some help here,' " Posey related, adding that he told the officer he wanted to see someone in the Salvadoran government "about sending assistance."

"He took my name and the next day they got back to me," Posey said. "I got a phone call saying I had an appointment with a Col. Mena or Menda, a logistics officer."

The Salvadoran officer turned out to be a Col. Mario A. Reyes Mena, Posey said, and "he spoke good English."

He said the American military officer who arranged the appointment with Mena left after introducing them.

"The individual that took me said he couldn't be present because he was an official of the U.S. government and couldn't get involved," Posey said. He said he could not remember the U.S. officer's name.

Together, Posey said, he and the colonel "drew up a list of small items I knew we could provide. They asked us who we were. I said I represented a small group of men that are getting together that want to do something."

Dressed in a a khaki CMA T-shirt with the motto, "Zero Hour for Communism!" Posey said in a lengthy interview here that he likes to set his sights high and at one point even told Nicaraguan rebel leaders "we had the objective of sending a million dollar's worth of supplies in a year's time."

The first shipment to El Salvador consisted of field equipment, packs, belts, canteens, first aid pouches, foot powder and similar items, Posey said. Smiling broadly, he said he sent it to the colonel through "the U.S. Post Office, insured."

He said CMA never sent large quantities of arms to El Salvador as it had first hoped to do, because it had never been able to raise enough money to buy them.

When Congress came through with funds for El Salvador, the CMA men turned their attention to the Honduran-based Nicaraguan rebels and were rewarded with the Nov. 25, 1983, invitation from Gen. Alvarez. "It's always good to hear from people like you, wishing to help with more than words," the general wrote Posey.

A few months later, in April 1984, the group decided to try to expand with an open meeting in a Decatur, Ala., restaurant and a guest speaker, Alfonso Callejas Deshon. He was a member of the FDN directorate and had been a Cabinet minister under the previous Somoza regime before he resigned in protest in 1972. A news item about the meeting in the Decatur Daily prompted a call from the FBI, Posey said.

"He the FBI agent called up from Huntsville and pulled out a lawbook with articles underlined about the Neutrality Act," Posey said. "I read 'em and said, 'Well, we're all right.' " The act prohibits Americans from launching invasions from U.S. shores of foreign countries with which the United States has diplomatic relations.

Posey said the interview lasted three to four hours. Ever since then, Blanton interjected, CMA officials have assumed that the FBI has been keeping watch on their activities.

"They the FBI kind of left it like the investigation would continue," Blanton said. "It's like we knew we were being up front and open and we were being monitored. That's the way we wanted it."