BOGOTA - The fourth anniversary of the crash and kidnapping of three U.S. defense contractors by leftist rebels in Colombia today might have passed unnoticed if not for the recent rescues of two Colombians kidnapped long ago.

The rescues of the two Colombians revived a never-ending debate here over the use of military force to try to free kidnap victims, running the risk they could be killed, negotiating a prisoner swap or paying a ransom.

A rescue attempt ''would be a death sentence for my son, his colleagues and the rest of the hostages,'' said Jo Rosano, the outspoken mother of Marc Gonsalves, a Florida Keys man captured by guerrillas along with Thomas Howes, who grew up in Cape Cod, Mass., and Keith Stansell, who has two children living in Georgia.

They were working for California Microwave Systems, a subsidiary of defense contractor Northrup Grumman, when their airplane crashed in southern Colombia. Their plane was locating clusters of coca crops, the raw material of cocaine. They are now the longest-held U.S. government hostages in the world.

Another U.S. citizen, Thomas Janis, and Colombian Luis Alcides Cruz were shot to death at the crash site. Three more U.S. contractors died two months later when their airplane crashed while searching for the three men.

As a policy, the United States says it does not negotiate with what it designates as terrorist groups, as the Colombian rebels are, and frustration over their captivity appears to be growing.

''Not a single day goes by that I don't personally think about their suffering and what I can do to speed their return,'' Southern Command chief Adm. James Stavridis said at a ceremony Monday marking the anniversary in Miami. ``Not a single day goes by that our interagency team doesn't search for evidence or track down leads that might lead to their rescue.

''I consider their rescue the No. 1 priority for U.S. Southern Command,'' Stavridis continued, adding that Southcom will soon have special wrist bands ''to symbolize our commitment and constant vigilance [and] . . . honor and remember'' the three.

RESCUE MISSIONS

Colombian President Alvaro Uribe approved rescue missions last year amid frustration over the fruitless quest to negotiate a prisoner swap between the Colombian government and the Revolutionary Armed Forces of Colombia, or FARC, the rebel group that is holding the contractors, dozens of soldiers and politicians and even a former presidential candidate, Ingrid Betancourt.

There are about 500 guerrillas in Colombian prisons and at least two top FARC members are facing prosecution in U.S. courts, one for drug trafficking and the other for the kidnapping of the contractors. The first trial of Ricardo Palmera on the kidnapping charges ended in a hung jury, but he is expected to face another trial soon.

Swaps have led to peace talks in the past, and polls show that the majority of Colombians approve of them. Momentum for a swap with the FARC seemed to be building last year until the rebels exploded a car bomb in Bogotá and Uribe called off the preliminary contacts.

Now momentum is building for military-led rescue missions because of recent successes.

Former Economic Development Minister Fernando Araujo escaped rebels after Christmas during a military sweep in northern Colombia, and security forces found army Capt. Leonard Moore chained to a tree in the central part of the country last week after an attack on guerrillas. They had been held six and four years, respectively.

But there are worries, because the FARC has killed high-profile prisoners in the past when pressured. Just four months after the U.S. contractors' airplane went down, FARC rebels executed 10 hostages, including a provincial governor and a former defense minister, during a rescue attempt in northern Colombia. In 2001, they executed a former culture minister in captivity as the army closed in on them.

''If you go there with guns blazing, a lot of people will probably get killed,'' said Steve Howes, Thomas' brother.

The FARC is thought to be holding the Americans deep in the southern jungles of Colombia, a foreboding area where the rebels have traditionally held sway. From the air, much of the region looks like an endless sea of trees, with the occasional river snaking into the distance.

SUCCESS UNLIKELY

One of the few outsiders to see the Americans, freelance Colombian journalist Jorge Enrique Botero, said in an e-mail exchange with The Miami Herald that the men told him in an interview six months after their capture in the FARC's jungle hide-out that they had walked for 23 days to reach their camp.

Botero said they were guarded around the clock by ''dozens'' of armed rebels and were being held in a small house of about 215 square feet. They left the room only to bathe and wash their clothes at a river. He says the guerrillas also move their prisoners constantly to avoid detection.

''The chances of a successful rescue are very low; I would say minimal,'' he wrote. ``They are always surrounded by armed guards, and access to the jungle zone where they are keeping them is very difficult.''

Botero recalled Stansell's words during an interview: ``This isn't Hollywood. If they come for us, we will surely die. These guards have automatic weapons, and we assume they know how to use them.''

His video interview with the three U.S. citizens is the last proof of life that family members say they have seen of the men.

Aside from Stavridis' comments at Southcom on Monday, the United States is not saying anything about the case and is even reticent about giving the men's ages and hometowns. Washington has backed Uribe with about $700 million in annual aid to help him combat drugs and the rebels, and it has supported Uribe's offensive tactics, which have led to a steady retreat by the guerrillas from urban areas.

Araujo's escape seems to have emboldened the Colombian government as well. The former minister slipped from rebel hands when the army attacked the camp where they were holding him. He then walked five days before stumbling into a village where army troops happened to be patrolling.

He told The Miami Herald via telephone that he was so desperate after six years in captivity that he preferred the risk of a rescue attempt.

''I was prepared for it,'' he said of the escape. ``I'd visualized it a thousand times.''

It's not clear if the three American captives feel the same as Araujo, but some of their family members do not want any government to decide for them.