As a police station in Athens is bombed exactly 100 days before the start of the Olympics, BBC News Online examines what is being done to provide security for athletes and the public attending the world's biggest festival of sport this August.
In security terms, Athens is seen as the most challenging Games yet
Who is responsible for the latest attacks?
While the timing of this latest attack is significant, Greece has been quick to insist there is no connection between the blasts and the Olympic Games.
The attack comes against a background of heightened fears over international terrorism, but police say they are investigating local left-wing extremist groups which have in the past planted bombs and carried out assassinations in Greece.
One group under the spotlight is Popular Struggle, which carried out a bomb attack in March, and threatened more violence.
But attacks by Greek terror groups have become less frequent since a government crackdown on the November 17 cell, which resulted in 19 of its members being jailed last December.
Correspondents say left-wing and anarchist groups oppose the Games, viewing them as a capitalist bonanza, and have protested at Western security services arriving en masse in Greece.
How big is the security challenge of the Games?
It is feared the huge profile of the Games - the first to be held since the attacks on New York, Washington, Bali and Madrid - could make Athens a prime target for terrorists.
The IOC has - for the first time - insured against partial or full cancellation of the Olympics, in a £93m policy covering terrorism, earthquakes, flooding and landslides
Greece is staging an unprecedented security operation at an estimated cost of 1bn euros ($1.2bn) - nearly four times what Australia spent for the 2000 Summer Games. Greek Prime Minister Costas Karamanlis has taken personal responsibility for preparation for the Games.
International Olympic Committee President Jacques Rogge says Athens 2004 will be the "most challenging" Games in terms of security. The IOC has - for the first time - insured against partial or full cancellation of the Olympics, in a £93m policy covering terrorism, earthquakes, flooding and landslides.
Greece has asked Nato to assist with air and sea patrols and has set up a seven-nation security advisory group. Agents from the FBI and Britain's Scotland Yard will also help police the Games.
A Greek delegation is currently in Washington, for talks on efforts to safeguard the games, amid US concern that construction delays at Olympic venues could undercut efforts for advance security testing and other procedures.
What measures are the authorities taking?
More than 45,000 security staff will guard the Games and escort teams to and from sports venues - three times as many as in Sydney.
Mark Spitz says the US could pull out at the last minute
American, British and Israeli athletes are to be given 24-hour armed guards. Competitors from the three countries are considered to be among the most likely potential terrorism targets. Guards will travel on athletes' buses, which could also be protected by armoured vehicles and police helicopters.
Athletes from other countries which have sent troops to Iraq may also receive similar protection.
In March, Athens held a two-week exercise to test security involving 1,500 Greek security personnel and several hundred US soldiers. The drill included a simulated chemical attack, a plane hijacking and an outbreak of an epidemic.
Next week, Olympic plans will be tested in a four-day simulation of an attack on the Games.
What reaction has there been to the latest attack?
Many countries are carefully monitoring the security situation in the run-up to the Games. France and Australia say they are reviewing security in the wake of the police station bombings. New Zealand said it was too early to say how the bombings would affect its participation.
More than 45,000 security officials will police the Olympics
The head of the Australian Olympic Committee confirmed Australia would send a team to Athens, but noted it was possible some individual athletes might pull out.
In the US, Mark Spitz, the seven times swimming gold medallist, said recently that security concerns could see the US withdraw from the Athens Games.
The US Olympic Committee denies a withdrawal is on the cards, but Spitz said any US decision to pull out of the Games would come at a late stage, and could trigger a "snowball effect" as other countries follow suit.
Have the Olympics been targeted before?
Munich in 1972 is infamous for an attack by Palestinian guerrillas that killed 11 Israeli athletes. But that attack came out of the blue, rather than from a climate of concern over international terrorism.
Consequently, Montreal in 1976 was one of the most heavily militarised Games on record.
A bomb attack at the Olympic Park in Atlanta Games in 1996, also killed one person.