KABUL -- Flags on Afghan government buildings and diplomatic missions around the world flew at half mast on Tuesday as the nation mourned for a murdered leader whose assassins may never be brought to justice, Reuters said.

Special services were held at mosques across the capital, Kabul, but it was business as usual in the city with government offices, shops and schools open as normal and the streets jammed with traffic.

Vice-president and public works minister Haji Abdul Qadir, a warlord businessman with powerful enemies, was shot dead in a hail of bullets in broad daylight on Saturday after his first morning in his new job as minister.

His murder has raised fresh questions about the ability of President Hamid Karzai to bring security to the war-torn country and unite the nation behind the fledgling government.

Karzai urged the nation to unite in a day of mourning for Qadir, but for many Afghans the struggle to eke out a living in one of the world's poorest country's took precedence.

"We work every day, even on Friday (day of prayers), but we still can't even meet our expenses," said Kabul bookshop owner Khair Mohammad. "I have to feed my children and cannot afford to close my shop. This stomach asks for food."

Qadir's two killers calmly climbed into a car and drove away after the shooting, prompting the arrest of several ministry guards -- appointed by his predecessor -- on the grounds they stood by and did nothing.

In all, 12 people have been detained in connection with the killing, but none are believed to have been directly responsible.

Karzai said the shooting would be fully investigated, and the Turkish-led International Security Assistance Force (ISAF) -- charged by the United Nations with enforcing security in the capital -- has accepted his invitation to join the investigation.

"The intention is to have a completely neutral, fair, quick and professional investigation for prompt identification of the culprits," said Karzai spokesman Sayed Fazl Akbar.

------------ ISAF Steps Up Patrols ------------ ISAF has stepped up security, with more helicopter patrols swooping low over the capital. The force did not say what expertise it could bring to the investigation, but the trail may anyway long since have gone cold.

Within hours of the shooting, journalists were allowed free access to Qadir's four-wheel-drive vehicle, its seats ripped up and covered in blood.

Any bullet casings or other forensic evidence were trampled on or destroyed underfoot.

The investigation of another recent assassination provides little hope that this one will be solved.

In February, then-tourism minister Dr. Abdul Rehman was gunned down at Kabul Airport.

Karzai named three members of the Northern Alliance -- the former opposition grouping of mostly ethnic minority forces that now makes up the core of the government -- as suspects and asked for their extradition from Saudi Arabia where they were on pilgrimage.

The three returned but none have been arrested.

Another Northern Alliance official Karzai alleged was behind the killing was suspended from office, but General Deen Mohammad Jurat has since been promoted to the position of interior ministry security chief.

Qadir, a former anti-Soviet Mujahideen leader in eastern Afghanistan, was given a state funeral in Kabul and buried in his powerbase of Jalalabad on Sunday.

The only thing that was clear in the investigation was that he could not have attained the power he had without making a lot of enemies -- especially within the lucrative drugs trade between eastern Afghanistan and Pakistan.

Qadir was one of the few ethnic Pashtuns in the Northern Alliance that swept the Taleban from power last year, backed by U.S. air power, prompting speculation his killing might have been the work of loyalists of the Taleban, which was largely made up of members of the majority Pashtun community.

His younger brother, Mujahideen Commander Abdul Haq, was executed by the Taleban when he was captured in eastern Afghanistan on a mission to rally opposition shortly after the United States launched air strikes last year.

The U.S. offensive was launched to flush out Islamic militant Osama bin Laden, prime suspect in the September 11 attacks, and punish his Taleban protectors.

ISAF spokesman Colonel Samet Oz would not say what specific help ISAF could give the investigation other than step up mobile patrols and coordinate with police. "Our main mission is to assist in maintaining security," he told a news conference. "But if we get some (specific) request, we will provide more security on this issue."

Turkey took over command of ISAF from the British last month.