Authorities are concerned that heightened law enforcement activity in Belgium and other parts of Europe have stirred up operatives with the Islamic State of Iraq and Syria, or ISIS, a U.S. government official told CBS News Thursday.

While investigators believe the deadly Brussels attacks were accelerated by Friday's arrest of the chief suspect in November's Paris attacks, Salah Abdeslam, U.S. investigators are working under the assumption that it's possible other similar plots may have been rushed into motion as well, CBS News homeland security correspondent Jeff Pegues reports.

The Brussels attacks were originally planned to take place this coming Monday but were moved up due to Abdeslam's arrest, a U.S. official told CBS News Thursday.

In Washington, U.S. Attorney General Loretta Lynch said Thursday there were no known links between the attacks and the U.S., CBS News justice reporter Paula Reid reports.

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During a press conference, Lynch and FBI Director James Comey acknowledged a concern that the attacks would inspire copycats, Reid reports. However Comey said he believes the pictures of the carnage may dissuade some people from joining ISIS because the pictures show the group isn't actually building a caliphate, but instead killing innocent people.

On Thursday night, Belgian authorities lowered the terror threat level one notch, but said the situation was "exceptional" and "grave" and that another attack was "likely and possible." The head of the terror threat assessment authority, Paul Van Tigchelt, said the imminent nature of the threat has lessened since the attacks. Nonetheless, he said "the danger has not gone away."

Belgium's interior minister and justice minister tried to resign Thursday ahead of an emergency meeting of European security chiefs held amid growing questions about why authorities couldn't prevent the attacks despite increasing signs of a threat.

Prosecutors announced a direct connection between the Brussels bombings that killed 31 people and injured 270 others and last year's attacks on Paris, which appear to have been carried out by the same ISIS network. The attacks have laid bare European security failings and prompted calls for better intelligence cooperation.

Interior Minister Jan Jambon said after a government meeting Thursday that "If you put all things in a row, you can ask yourself major questions" about the government's handling of the threat from Islamic extremists.

Notable among the questions is those raised by Turkey's announcement that it had warned Belgium last year that one of the Brussels attackers had been flagged as a "foreign terrorist fighter."

But the prime minister asked Jambon and Justice Minister Koen Geens to stay on, given the current challenge the government is facing.

The meeting came as Belgian and French media reported a second attacker is suspected of taking part in the bombing this week of a Brussels subway train and may be at large.

The U.S. official told CBS News that Belgian police were trying to locate a second man seen at the location of the subway bombing. The man was seen buying tickets in the subway with the confirmed suicide bomber, Khalid El Bakraoui, and both men had identical backpacks, the U.S. official said.

Belgian prosecutors have said at least four people were involved in Tuesday's attacks on the Brussels airport and a subway train, including El Bakraoui and his brother Ibrahim, who has also been identified as a suicide bomber. European security officials identified another suicide bomber as Najim Laachraoui, a suspected bombmaker for the Paris attacks.

Khalid El Bakraoui blew himself up on the train, while Ibrahim El Bakraoui and Laachraoui died in the airport.

Prosecutors have said another suspected participant in the airport attack is at large, a man in a hat seen in surveillance images who has not been publicly identified.

Prosecutors, who have not said how many people overall may have taken part in the bombings, did not respond to the reports.

The federal prosecutors' office issued a statement Thursday saying that Khalid El Bakraoui had rented a house used as a hideout for the Paris attackers, and that he had been hunted by police since December.

Several of the Paris attackers were Belgian or had links to Belgium, and the country has been on high alert for possible attacks.

Turkey's president said Wednesday that one of the Brussels suicide bombers, Ibrahim El Bakraoui, was caught in June 2015 near Turkey's border with Syria and deported to the Netherlands, with Ankara warning Dutch and Belgian officials that he was a "foreign terrorist fighter." Turkish officials said he was later released from Dutch custody due to lack of evidence of involvement in extremism.

European Union justice and interior ministers were holding an emergency Thursday afternoon to discuss the attacks, and French President Francois Hollande said France would "speak loud and clear" for better intelligence sharing and tougher measures against weapons trafficking.

Also Thursday, Abdeslam was summoned to court in Brussels after his arrest last week in the Belgian capital. His lawyer, who had initially vowed to fight extradition, said Abdeslam now wants to be sent to France as soon as possible.

Abdeslam evaded police in two countries for four months before Friday's capture. Abdeslam's lawyer, Sven Mary, told reporters at the courthouse that he asked for a one-month delay on any transfer while he studies the large dossier, but that Abdeslam "wants to explain himself in France, so it's a good thing." Mary said the extradition process should be done by mid-April.

France is seeking Abdeslam's extradition to face justice for his involvement in the Nov. 13 attacks on a Paris rock concert, stadium and cafes, which killed 130 people. Several attackers were also killed.

Belgium is holding three days of national mourning.

Belgian Prime Minister Charles Michel, in a speech Thursday, said the attacks on the European Union's capital targeted the "liberty of daily life" and "the liberty upon which the European project was built."

"Our country and our population were hit at its heart," he said in front of the Parliament building. He honored the "children who have lost their papas, who have lost their mamas" in the attacks.

Security remains tight, but barriers were removed around the subway station hit by the attack, Maelbeek. The airport will remain closed until at least Saturday.