Belgian state broadcaster RTBF and France's Le Monde and BFM television reported on Thursday that a fifth attacker may also be at large: a man filmed by surveillance cameras in the Brussels Metro on Tuesday carrying a large bag alongside Khalid El Bakraoui.
The suspects have not yet been charged, pending a decision on Friday.
The Brussels attacks came days after police arrested Salah Abdeslam, a suspect in the Paris attacks.
The French Interior Minister announced Thursday that raids in northwest Paris foiled a planned attack as two suspects in the deadly Brussels bombings remain on the loose.
The suspect arrested in Argenteuil, a northwestern suburb of Paris, on Thursday is likely an important member of a terrorist network, French Interior Minister Bernard Cazeneuve said.
Abdelsam is also linked to Najim Laacharoui, one of the two suicide bombers of Brussels airport, per reports.
More details have emerged over exactly what the security services knew about the terrorists involved in the atrocities. Because Bakraoui was not on any watch lists at the time and because he had a valid Belgian passport, van der Steur said, "there was no reason to take any action".
Footage posted on social networks showed the man lying near ytrain tracks, clutching a backpack, as a bomb disposal robot inched forward.
Kerry landed at the still-closed Brussels airport for a brief, hastily scheduled stop from Moscow, where he said the attacks underscored the urgency of unity in the fight against the Islamic State group.
Kriket is believed to have rented the Argenteuil apartment, Mothron said.
Belgian Interior Ministry
Student protests were held across France on Thursday against proposed labour reforms which saw cars torched in the capital and clashes with riot police, who responded with tear gas and made more than 30 arrests.
A security guard helps a wounded woman outside the Maalbeek metro station in Brussels.
Arriving at the meeting, Mrs, May said the killings were "cold-blooded and sickening attacks".
Geens said systems should be reviewed but noted that other countries had been attacked, citing September 11, 2001, attacks on the United States in which he noted that "there were 3,000 dead".
The workers - who wore fluorescent vests emblazoned with the words "We are working to protect and preserve your messages" - stepped gingerly among the flags, tea candles and beer bottles left as tokens of support, slowly picking the sopping hand-written notes and stacking them into the bins.
At least four Americans in Brussels have not been seen since the attack, including citizens Justin and Stephanie Shults, who have been living in Belgium since 2014.
Mourad Laachraoui, Najim Laachraui's younger brother, said that he is overwhelmed with the evil his twisted sibling unleashed, telling reporters that "I feel bad, that's all - scared and saddened".
Their family was a practising Muslim household, but he could not say what put his brother on the path to violent extremism.
"I am not excusing it but this could explain things", he said.