WASHINGTON -- He is rarely quoted in the newspaper or shown on television. He has never given an interview and apparently has never delivered a speech in public. He is said to stutter.

Most Iraqis, it is said, would not recognize the short man with the thick mustache if they encountered him on the street--though two who did tried to kill him recently, according to an opposition group.

Qusai Hussein, 36, the younger son of Iraqi dictator Saddam Hussein, has emerged as a significant figure in the regime and an object of growing U.S. concern. He commands key military, security and intelligence forces and, U.S. officials say, directs lucrative smuggling networks in violation of United Nations sanctions.

If an invasion killed or dislodged his father, Qusai could well be left in control of the regime's deadliest weapons.

A British government report revealed last week that the elder Hussein may have delegated to Qusai authority over Iraq's suspected arsenal of chemical and biological weapons. The disclosure was the latest sign that Qusai has eclipsed his infamous older brother, Uday, as the Iraqi strongman's most trusted aide and heir apparent.

U.S. officials say Qusai's growing power has emerged as a wild card as U.N. inspectors plan to return to Iraq to search for possible weapons of mass destruction--and as the Bush administration weighs how best to topple Hussein's regime.

"If Saddam is knocked out early, and command and control breaks down, then how do they fire their stuff?" said a senior U.S. intelligence official who is involved in the planning. "Who's going to push the button?

"Qusai is someone who, A, would be loyal to his father, and, B, if he gives the orders, those orders will be followed like his father's," the official added. "There's contingency planning going on, and he's a big part of it."

According to U.S. and British intelligence, Hussein's regime in recent years has produced both biological and chemical weapons and can deliver them with artillery shells, free-fall bombs, sprayers and ballistic missiles. They could be deployed within 45 minutes, officials say.

Would Qusai use them? Former U.N. weapons inspector Terrence Taylor isn't sure. "These are not Taliban, theologically motivated people," said Taylor, now president of the International Institute for Strategic Studies-U.S. "This is a secular ruling clique, a Mafia-type group. They want to survive. So they're going to try to strike deals."

Qusai had no known role in his father's past military adventures, including the war against Iran in the 1980s and the invasion of Kuwait in 1990 that led to the Persian Gulf War the following year.

But Qusai knows about Iraq's clandestine weapons programs. Iraqi defectors have told U.S. officials that starting in the mid-1990s, he headed a special unit of as many as 2,000 men whose job was to hinder and hamper U.N. weapons inspectors.

According to these accounts, one team from Qusai's group would create traffic jams and other diversions to delay the inspectors, while another team would rush to move incriminating records, equipment and other items that the inspectors were seeking.

David Kay, a former U.N. weapons inspector, says Qusai can be seen "lurking in the background" in photographs of key inspections. Another former inspector, who asked not to be identified, says Qusai was part of a high-level Iraqi committee that "decided what to give up and what to conceal. He was involved up to his eyeballs."

U.N. inspectors withdrew from Iraq in frustration in December 1998 and have yet to return.

Experts say Qusai's influence has grown dramatically since then, especially in the military and security structure of the Iraqi police state. Getting to the top wasn't hard; his father appointed him to every post.

Qusai oversees the Republican Guard, the best trained and armed military unit, and the Special Security Organization, which is entrusted with protecting the president and with hiding any weapons of mass destruction. The SSO also monitors telecommunications between Iraq and the outside world, Iraqi exiles say.

The two posts, as well as a recent appointment as head of the northern army, the force that presumably would defend Baghdad against an attack from Kurdish areas in the north, give Qusai operational control over some of the most important units in Iraq's armed services.

Qusai also helps run the Mukhabarat, Iraq's largest and most dreaded intelligence and internal security service.

The U.N. Commission on Human Rights, Amnesty International and other human rights groups have accused the Mukhabarat of torturing suspected dissidents or their families.

A former Mukhabarat member, Khalid Janabi, told U.N. investigators last year that members of a special unit, the Technical Operations Directorate, have raped relatives of suspected opponents and then used videotape of the assaults to ensure future cooperation.

Qusai's public profile increased dramatically in May 2001, when he was elected in a secret ballot--and his brother was not--to the ruling Arab Baath Socialist Party's leadership council at a party conference in Baghdad.

Not everyone was pleased. According to a U.N. report, Iraqi security forces executed a leading Shiite Muslim cleric, Hussein Bahar Uloom, for refusing to "publicly express approval" of Qusai's election. Human rights observers say the killing was part of a campaign to eliminate Shiite clerics opposed to the secular Baghdad regime.

Still, the state-run television announcement of Qusai's election reportedly caught many Iraqis by surprise. He has shunned the spotlight, remaining a behind-the-scenes player in the daily deadly intrigue of Iraqi politics.

Judith Yaphe, a former CIA analyst and an Iraq expert, says Hussein may yet undermine Qusai. "He doesn't want his son to have too much power and authority because he might think, 'Gee, if I got rid of Dad ....' If you're Saddam Hussein, you have to think like that," she said.

Other analysts say Qusai's rise has fueled feuds among the Iraqi elite.

On Aug. 1, two men in an Oldsmobile shot and wounded Qusai in the arm as he rode in a convoy through Baghdad's Mansour district, according to the Iraqi National Congress, an opposition group based in London. The assailants were killed at a military roadblock, the group says, and their identities remain unknown. The CIA says it cannot confirm the incident.

Trained as a lawyer, Qusai is said to live a quiet life, devoted to his wife and three sons. One Iraqi defector has said that he is a heavy drinker, but U.S. officials say the evidence is not clear.

"He's viewed as level headed, restrained, not a wild man," said Kenneth Katzman, an Iraq expert at the Congressional Research Service.

"He's seen as stable, low profile, very loyal to his father," said an intelligence official.

Qusai is also viewed as a shrewd player in Iraq's $2-billion-plus black-market economy.

Analysts say Qusai and the government agencies he runs have taken control of much of Iraq's illicit sales of crude oil and diesel fuel to Turkey, Lebanon and especially Syria, according to a report by the Coalition for International Justice, a Washington-based research group that has studied Iraq's economy. The sales violate U.N. trade sanctions imposed after the Gulf War.

"He's really become a major player over the last 18 months to two years," said Susan Blaustein, co-author of the report.

Defectors have said that Qusai's Mukhabarat has set up front companies in Jordan that re-export goods legally imported into Iraq for civilian use under the U.N. "oil for food" program--goods such as medicine, baby food, vehicles, spare parts and electronics. The intelligence agency also has muscled into the lucrative trade in black-market cigarettes from Cyprus, the coalition says.

"Qusai is likely able to make good use of the money, not so much to support a lavish lifestyle as to retain the loyalty of the thousands of members of his various services," the coalition report says.

Qusai's rise to power reflects, in part, the decline of his older brother, Uday. Perhaps the only Iraqi more feared than his father, Uday was shot and severely injured in a 1996 attempted assassination.

Uday, 38, is Iraq's media baron, a flamboyant figure who races a red Porsche and controls Iraq's most popular radio and TV stations and its largest newspaper. He heads student groups, the bar association and the Iraqi National Olympic Committee. Iraq wants to bid to host the 2012 Summer Olympics, and the U.N. in June approved spending $20 million generated from legal sales of Iraqi oil to begin building a 100,000-seat stadium in Baghdad.

Uday also is known for his violent temper. Human rights groups say Hussein's elder son runs a black-garbed paramilitary force called Saddam's Martyrs that has torn the tongues out of supposed enemies and used swords to execute victims outside their homes.

He also allegedly has maintained a private torture chamber, known as the Red Room, in a building by the Tigris River. He has used whippings, beatings and immersion in sewage to punish Iraqi athletes who lose, according to two national team soccer players who defected.

Hussein's sons "differ only in that Uday kills people for fun and Qusai kills people in a very busi-nesslike fashion," said former CIA chief R. James Woolsey. "If Saddam dropped over of a heart attack tomorrow and we were dealing with Qusai, we would be no better off."

Times staff writer Sonni Efron contributed to this report.