As police tightened security at public buildings and airports across the United States, authorities announced on Friday that they have located the flight data and voice data recorders - called "black boxes" - from a hijacked airliner that crashed into the Pentagon on Tuesday.
The recorders could provide clues to what happened in the cockpit of the American Airlines jet that was commandeered on Tuesday shortly after it took off from Dulles airport, heading for Los Angeles with 64 passengers on board.
Four commercial airliners were commandeered; two slammed into twin towers of the 110-storey World Trade Center in New York, and one, into the Pentagon. The fourth crashed in woods near Pittsburgh, Pennsylvania. The flight data recorder from that plane was recovered earlier.
US Attorney General John Ashcroft said at least 18 hijackers were involved on Tuesday.
The United States has publicly named Osama bin Laden as a suspect in the terrorist events. Secretary of State Colin Powell, in televised comments, said the millionaire Saudi dissident was a prime suspect, but he emphasised that authorities are also investigating a number of suspected terrorist groups.
The investigation continued to widen, as police in a number of countries joined in the search for the perpetrators of the attacks.
In Germany, police detained, and then released an airport worker. They said two other men believed connected with the attacks had lived in Hamburg, including Mohamed Atta, 33, who was a passenger on one of the hijacked planes.
Aircraft on alert
Across the United States, US military aircraft are on "strip alert", prepared to mobilise within minutes if they are required to protect US airspace, according to Defence Secretary Donald Rumsfeld.
The US Senate, meanwhile, has given President George W Bush the go-ahead to "use all necessary and appropriate force" in retaliation for the attacks. The House of Representatives is expected to follow suit on Saturday.
Limited air traffic was restored at US airports, with severe security measures that required long waits for passengers, and an end to such conveniences as curbside check-in. As passengers headed for planes, specially trained security dogs patrolled the runways.
President Bush planned to travel to New York on Friday to show his support for the ongoing recovery effort, including the search for victims.
Nearly 5,000 missing in New York
New York Mayor Rudy Giuliani says 4,763 people are missing from the twin towers of the World Trade Center. Officials put the death toll at 94, and said 3,800 were injured; however those figures were expected to change as the recovery process continues. At the Pentagon, 126 people remained unaccounted for, officials said.
As emergency teams, firemen, FBI investigators, and others laboured at the debris-strewn crash sites, men and women clutching photographs of missing loved ones stopped reporters, politicians and camera crews in New York, begging for help in finding them. Many had made repeated visits to hospitals, seeking news.
Meanwhile, the 50-story Liberty Plaza, located near the trade centre, has become more unstable and could collapse, adding to the danger and difficulty of the cleanup. A number of skyscrapers housing some of the world's most important financial institutions had broken windows, and lobbies and offices covered with soot from the fallout of the collapsed buildings.
In pockets of the financial district of lower Manhattan, thousands of utility workers are attempting to restore communications and electricity, which were cut off following the explosions and collapse of the trade centre towers.
U.S. stock markets are closed on Friday, but are expected to reopen on Monday. In a move to keep the global economy stable following the terrorist attacks, the US Federal Reserve is making $50 billion (SFr82.55 billion) available to stabilise European banking systems.
Denial of involvement
Bin Laden has denied involvement in the attacks, according to a spokesman in Pakistan. However, he described them as "punishment from almighty Allah."
In Pakistan, which borders Afghanistan, where bin Laden is believed to be sheltered, the army closed Islamabad International Airport to commercial flights for more than two hours overnight, to transport military equipment, officials said.
The Afghan capital, Kabul, was reported to be tense, as many residents there have expressed the fear that the United States may target the city for military action, if the investigation provides evidence of a strong link to the terrorist attacks.
swissinfo with agencies