Belgian prosecutors say six people have been detained in raids around Brussels linked to this week's attacks on the city's airport and subway system.

The announcement came after Belgium authorities lowered the country's terror threat level one notch, while noting the situation there is "exceptional" and "grave" and that another attack is "likely and possible."

Federal prosecutors said in a statement late Thursday that the detentions were made during raids in central Brussels, Jette and the Schaerbeek neighborhood. Police found a large stash of explosives and other bomb-making material earlier this week in an apartment in Schaerbeek believed used by the suicide bombers.

Schaerbeek residents described hearing detonations during the police raids. It was unclear whether they were explosions or controlled detonations.

Earlier, the head of the terror threat assessment authority, Paul Zan Pigchelt, said the imminent nature of the threat has lessened since the attacks on the airport and subway Tuesday.

Nonetheless, he says "the danger has not gone away."

Thirty-one people were killed when suicide bombers targeted the Brussels international airport and a Metro station on Tuesday. Another 316 people were injured in the blasts, according to officials who spoke to NBC News Thursday. A spokesman with Belgium’s Foreign ministry confirmed about 40 nationalities have been registered for the wounded, while the nationalities of the dead are still trying being determined.

Prosecutors have said at least four people were behind the attacks. Officials identified Najim Laachraoui and two brothers — Ibrahim and Khalid El Bakraoui — as suicide bombers. A fourth attacker is suspected to be at large.

The Dutch justice minister has confirmed that one of the Brussels suicide bombers was flown from Turkey to Amsterdam in July, but says that authorities weren't told why and had no reason to detain him.

In a letter to parliament, Justice Minister Ard van der Steur said Thursday that Ibrahim El Bakraoui was put on a plane from Istanbul to the Dutch capital on July 14, but that Turkish officials didn't say why and his name wasn't flagged in any Dutch law enforcement databases.

Van der Steur says that El Bakraoui had a valid Belgian passport when he arrived in Amsterdam "so there was no reason to take any action" at Schiphol Airport.

It wasn't clear what El Bakraoui did after arriving in the Netherlands.

President Recep Tayyip Erdogan said on Wednesday that El Bakraoui, was caught in June 2015 near Turkey's border with Syria and deported, at his own request, to the Netherlands, with Ankara warning Dutch and Belgian officials that he was a "foreign terrorist fighter."

The Dutch version of events appeared to contradict that, with Van der Steur saying that an electronic message from Turkey's foreign ministry to the Dutch embassy in Ankara gave no information about the reason El Bakraoui and an unidentified German national were put on the flight,