Jorge Martinez last saw his daughter, Olga Cruz, six years ago when he left El Salvador and came to Washington as an illegal alien. When he got a collect call from Texas a little past noon Tuesday, he thought it would be Olga saying she had safely slipped across the Mexican border and would soon be joining him.

Instead, a man named "Reuben" told Martinez that his 15-year-old daughter was in a Texas hospital with her cousins, Alva Merino, 13, and Gloria del Carmen Velasquez, 16. Another cousin, Sandra Merino, 15, was missing and possibly dead.

Martinez and his relatives had arranged for their daughters to be smuggled into this country, at $1,400 each, and for a moment it seemed the risk they had taken had been too great.

But with the help of a lawyer, Sandra Merino was eventually located alive. And despite their injuries, Olga and her three cousins can count themselves lucky. Four of their companions, found Monday in an airless tractor-trailer parked on Texas Rte. 281, had suffocated.

Every day thousands of illegal aliens, many of them fleeing bloody civil wars in El Salvador and Guatemala, seep through U.S. borders to join relatives and friends. Many are helped by "coyotes" or professional smugglers who are usually part of a sophisticated chain extending from Central America to the urban centers of this country.

The trip that nearly ended in tragedy for the four cousins began last month when a woman approached Blanca Merino, Jorge Martinez's sister, at a bus stop in Washington. Merino, who came here illegally five years ago, holds down two jobs. By day she works at the Watergate Bakery and at night she cleans offices.

The woman engaged Merino in conversation, asking where she came from; whether she had a 'green card' that allows her to work legally and where her children were. When she heard that Merino's daughters were still in El Salvador, she said she knew someone who could help her and took her telephone number.

Soon afterwards, Merino told immigration officials yesterday, a man who identified himself only as "Rocaleo" called and said he would come to see her. When he arrived at her Adams-Morgan apartment, he set out the terms: $1,000 in cash immediately and $1,800 more to be paid once Alva and Sandra arrived in Washington. (The initial fee rises to $1,500 if the alien is over 18 years.)

Merino paid the money and put "Rocaleo," whom she described as a thin, blond, curly-haired Latin in his early 30s, in touch with Martinez, a restaurant busboy at the L'Enfant Plaza Hotel, and Maria Eulalia Velasquez, a cleaning woman who is Merino's sister-in-law. They both agreed to pay $1,400 when their daughters were delivered to them. No down payment was required of them.

Merino said yesterday she wanted her daughters here for "two reasons: so they could get a good education and so they would not be killed." She said they had been riding on a bus with their grandfather last year in the rural district of San Miguel where they lived and were stopped by "unknown men" who were armed. The men separated all those who had identification papers from those who did not. Those who did not were taken away and have not seen since, Merino said.

One of her daughters was among those to be taken away and was left behind when her grandfather protested vigorously. After that incident, Merino said, she began to save money to bring her daughters to Washington.

Martinez and Velasquez, who has been here two years, tell similar stories. Martinez said he feared for his daughter's life when he got a letter from her grandmother about six months ago saying that Olga was no longer living with her because on two occasions "uniformed men" came looking for her. Velasquez, who is the mother of seven children, only two of whom are with her in Washington, said she became worried after she heard that the guerrillas fighting the Salvadoran government came to her children's school and killed a teacher and wounded a student.

According to Jim Kerr, assistant chief of the U.S. Border Patrol at McAllen, Tex., a group of 50 Salvadorans left their country last week in a chartered bus. When they arrived in Reynosa, Mexico, the group was divided into two. Gloria del Carmen Velasquez and her three cousins were placed in a group of 13 females and 13 males.

Sunday night they were taken across the Rio Grande River, probably by boat, according to Kerr. Once they were in Texas, they were put in a refrigerated tractor-trailer used for transporting vegetables. The tractor rig had Ohio plates and the trailer was registered in Maine, Kerr said.

Sometime on Monday the truck was driven "north of Pharr, Tex., and parked at an underpass on Highway 281" about 20 miles from the Mexican border, Kerr said. The Salvadorans, who ranged in age from 14 to the 40s, were brought food once on Monday, while the "coyotes" apparently waited for dark to drive farther north.

Around 7 p.m., according to accounts given by the Salvadorans, the driver of the tractor-trailer opened the back of his vehicle and found most of his human cargo suffocating. Four were already dead. The air-conditioning had failed and in the 90-degree heat, the trailer had become a death trap. Many of the Salvadorans had shed their clothes in the heat.

Kerr speculates that the "coyotes" panicked. They ordered 10 aliens who could still walk out of the truck, packed them into a small van that had accompanied the truck, and drove them to a citrus grove three miles away. There, near Edinburg, Tex., they were dumped off, some of them wearing only their underwear. When they began going to nearby houses seeking clothes, the U.S. Border Patrol was contacted and seven members of the group were apprehended. Sandra Merino was in this group.

Twelve others still alive in the truck, among them Alva Merino, Cruz and Velasquez, got out and flagged down a motorist. They were taken to nearby hospitals by the sherriff's office and Border Patrol. There they met "Reuben," an evangelical minister, who contacted their parents here.

Yesterday, two men from Pharr, Tex., turned themselves in to the U.S. Border Patrol in connection with the incident, according to a spokesman in the Hidalgo County sheriff's office. The two men, identified as Robert J. Manners, 51, and Edward G. Hunter, 38, were brought before a U.S. magistrate and charged with transporting or conspiring to transport illegal aliens into the United States, the spokesman said. They are being held in the Hidalgo County Jail in Edinburg.

A warrant has been issued for the arrest of a third man, 38-year-old Larry David McCoy, the spokesman said. All the apprehended Salvadorans who are no longer in the hospital are being held in the jail also as material witnesses for the federal government, according to the spokesman.