Brussels (CNN) First France, now Belgium and possibly Greece. Where next?

The recent spate of terror attacks and threats in Europe has many wondering what the next target might be and how the danger can be mitigated.

Here are the latest developments:

Belgium: More suspects arrested

Five Belgian nationals have been charged with participation in a terrorist organization after police raided a suspect terror cell in Verviers, the country's federal prosecutor said. Three are in custody, and two have been released "under strict conditions," Erik Van Der Sypt said.

And two additional suspects are in French custody after they were detained while trying to cross from France into Italy through the Frejus Tunnel, the prosecutor said.

The suspected terror cell in Verviers, which included people returning from Syria, planned to target police officers, Van Der Sypt said. Police said they recovered weapons, bomb-making materials and police uniforms.

Two suspects were killed in the police raid Thursday, and a third was injured and taken into custody. Authorities have not released their identities.

Heavily armed guards filtered through the streets of Brussels and Antwerp on Saturday, a sight the country hasn't seen in 35 years.

Why is Belgium particularly vulnerable? The small country is believed to have more suspected jihadists per capita than any other country in Western Europe.

Greece: Police join Belgian investigation

Greek authorities say they are cooperating with Belgian authorities on an investigation, and on Sunday Belgium requested the extradition of one of the suspects arrested in Greece.

Greek police confirmed that there have been some arrests and are looking for other individuals, but said that none so far were linked to the Belgian plot.

Belgian federal prosecutor Van Der Sypt, however, said the Belgian investigation "gave us enough reason to ask for the extradition of one of the persons that were arrested yesterday by the Greek authorities."

The investigation revealed that one of the people arrested in Greece could be involved in the plot that was stopped in Belgium, Van Der Sypt said.

Across Europe: Fear of 'sleeper cells'

European counterterrorism agencies are scrambling to assess the potential terrorism threat from people with suspected links to Islamic extremists.

As many as 20 sleeper cells of between 120 and 180 people could be ready to strike in France, Germany, Belgium and the Netherlands, a Western intelligence source told CNN.

European Union and Middle East intelligence agencies identified an "imminent threat" to Belgium and possibly to the Netherlands, the source told CNN.

"There is a tremendous amount of concern over sleeper cells in Europe," said a Western official with direct knowledge of the situation.

But while there is a "realistic threat" to the Netherlands, government spokesman Edmond Messchaert said there is "no concrete or specific information of an attack in the Netherlands."

France: DNA links two to market attacker

Two additional people whose DNA was found on the belongings of jihadist Amedy Coulibaly are being sought by authorities, according to a source in Paris with knowledge of the investigation into this month's attacks in the French capital. The DNA of one was found on a charger belonging to Coulibaly, the source told CNN.

Coulibaly is accused of killing four people in a siege at a kosher market and gunning down a police officer.

Meanwhile, three women detained in connection with the Paris terror attacks were released from custody Sunday, the public prosecutor's office told CNN.

Nine other people will remain in custody for at least 48 hours while the investigation continues.

The detainees were held after a series of terror attacks this month that left 17 people dead -- including 12 people in an attack at the offices of the Charlie Hebdo satirical newspaper.

Al Qaeda in the Arabian Peninsula, al Qaeda's Yemen affiliate, claimed responsibility for the Charlie Hebdo shooting.

The publisher of Charlie Hebdo says the newspaper will increase the press run of the latest issue, which features a depiction of the Prophet Mohammed, to 7 million.

ISIS: A threat from abroad

ISIS, the extremist Islamist group that controls swaths of Iraq and Syria, runs large training facilities in Syria and has access to thousands of potential European recruits.

Officials are monitoring groups of men who have returned to Europe from fighting in Syria.

It is believed ISIS directed operatives to return to Europe to launch attacks in retaliation for airstrikes against the terror group in Syria and Iraq, according to the official.

Several European nations, including the United Kingdom, France, Belgium and the Netherlands, are participating in the air campaign against ISIS in Iraq.