Mario Calero was somber as he read from the "sort of diary" that he said was kept by one of two Americans killed in a rebel helicopter in Nicaragua last Saturday.

"These people, and the Americans I am with, make me proud to be here," James P. Powell III of Memphis wrote shortly before he was killed.

"I think of my son . . . my mother and father and all my friends coast to coast. They probably don't understand why I am here. I am sure I am doing what I think is right," he wrote.

Powell and Dana H. Parker of Huntsville, both 36 and Vietnam veterans, died in what colleagues described with admitted uncertainty as some type of hurried "emergency mission" during a rebel attack on a military school run by Nicaragua's Sandinista government near Santa Clara, Nicaragua.

According to friends and colleagues who held a news conference here tonight while others consoled grieving relatives, Parker and Powell were showing a Nicaraguan companion, Mario Pozo, how to fly the helicopter with its rocket pods attached when they received a message that caused them to fly from their secret rebel base abruptly at about 1:15 p.m. on Saturday.

They said that the helicopter returned and that Parker dashed out to shout a brief explanation to another American, identified only as "Bill," before the craft took off again.

"We weren't sure, because of the turmoil, whether they said 'emergency mission' or 'emergency rescue mission,' " said Walter (Cisco) Blanton of Sheffield, Ala., one of four other Americans who accompanied Parker and Powell on the trip to assist the contras, as the anti-Sandinista rebels are known.

Calero, a Louisiana-based official of the Nicaraguan Democratic Front (FDN) who accompanied the Americans, acknowledged in frustrated tones that several questions remain unanswered.

He said, for instance, that earlier in the day the two men had been showing Pozo, who was killed with them when the helicopter was shot down, how to fly it when it was equipped with stretchers.

When they took off for the last time, he said, "it did have the rocket pods on . . . . I believe the stretchers had been taken off."

"None of us knew, with the exception of 'Bill,' that they had gone on this rescue mission," Calero said at another point. He said most of the rest of the group of six Americans and himself was preoccuppied at the other chores at the rebel camp.

"Bill" was not at the news conference and was said to be with Parker's former wife and two daughters as well as other relatives.

Calero and others at the session in the Huntsville Hilton were emphatic about several points, however.

They said Powell, a helicopter pilot shot down three times in Vietnam, and Parker, a Marine enlisted man in South Vietnam, had "no sidearms" or weapons with them on their last flight. They said they received not "one penny" from the U.S. government or any other source for their unpaid volunteer efforts.

They said they resent being called "mercenaries."

"Get a dictionary out," declared Tom Posey, a produce dealer from Decatur, Ala., who founded the Civilian-Military Assistance group to which the Americans belonged. "A mercenary is one who fights for money . . . .These men did not get paid anything but beans and rice. The label should be 'freedom fighter.' "

The CMA and a sister group that concentrates on helping refugees were founded 14 months ago, according to Posey and Calero, and began supplying the FDN rebels with clothing, boots, medical supplies, toys and other such materiel last January.

They said about 15 Americans, including the group of six, have traveled to Central America at their own expense to show contras how to shoot and provide them with other "military expertise."

Calero said his rebel group has been given about $70,000 in supplies but "no weapons whatsoever" since the effort began.

"We are trying to get the bodies . . . . We would like for the [Nicaraguan] government to let us know if there will be a ransom demanded so . . . we can raise the money," Posey said.

"Their bodies and souls demand it," Posey said of Parker and Powell. "They gave their lives, and their lives tell us it's worthwhile. It's not just the Sandinistas. It's communism . . . ."

Calero ended with a poignant footnote about Parker, who embarked with most of the others by commercial airliner from New Orleans in late August.

"He took a stack of masks, cartoon-type masks, with him from a restaurant in New Orleans to give to the kids," Calero recalled. "One of the first kids who got those masks had taken part in 25 combats. He was 10 years old."