BRUSSELS (AP) — Belgian authorities are investigating information that suggests the Islamic State group has sent more fighters to Europe, an official said Tuesday, as Belgium remains on high alert following last month's suicide bombings in Brussels.

"Signals appear to suggest that IS has again sent fighters to Europe, to our country," Paul Van Tigchelt, the head of Belgium's security threat analysis center, told reporters in Brussels Tuesday. He did not elaborate.

Belgium has been on at least its second highest alert level for four months — since the week after the November 13 attacks in Paris — with troops and extra police mobilized.

Strategic sites like airports, major rail stations, nuclear plants and buildings housing radioactive materials remain under close surveillance, while security is high at soft targets like shopping complexes and cinemas.

Federal Police spokesman Peter De Waele said surveillance cameras are being monitored full time and that car number plate reading devices are in use around the airport. More than 50 suspect packages have been dealt with since the Brussels attacks, he said.

"The threat is still there, it's serious and possible," Van Tigchelt said. "We have to stay alert."

Belgian authorities believe they have "destabilized" the network that carried out the March 22 suicide attacks on the Brussels airport and subway, killing 32 and injuring hundreds. The perpetrators have been closely linked to the group that carried out the attacks in Paris.

Van Tigchelt said the continued high alert level was not based solely on the Paris-Brussels network, some of whose acts were prepared in the Brussels suburb of Molenbeek, where several extremists have lived or traveled through.

"There are several ongoing investigations," Van Tigchelt said. He declined to be drawn on the exact nature of the threat.