The suicide bombings in Brussels represent a shift in tactics for the Islamic State terrorist group toward direct, mass casualty attacks in Europe, according to a State Department security report.
"The March 22 bombings in Brussels and the November 13 attacks in Paris highlight a strategic shift by ISIL to direct operations in Europe versus relying on inspired and self-radicalized individuals," said the report by the Overseas Security Advisory Council, a State Department security office that works with American corporations overseas.
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"European authorities continue to warn of the risk of additional mass casualty attacks in the region," the internal report based on open sources said.
Additionally, dozens of active Islamic State terrorists are now operating in Belgium and Europe. The Tuesday suicide bombings were carried out by Belgian extremists trained in Syria and Iraq who returned to the continent, the report said.
Three suicide blasts—two at Belgium’s international airport and one at a subway station— were set off between 8:00 and 9:00 a.m. local time, killing 31 people and wounding 270.
The report warned that the threat of further terrorist attacks in Europe "remains high in the near future."
"So far, ISIL has not targeted the U.S. private sector in Europe," the report said. "However, the group is likely to continue coordinating attacks against soft targets to heighten the potential for collateral damage and maximize causalities."
As a result, Americans in Europe should be vigilant for further terrorist bombings or shootings.
The report said two of the terrorists were brothers Khalid and Ibrahim El Bakraoui.
A third bomber who blew himself up at the airport has not been identified. Police, however, are looking for Najim Laachraoui, an ISIS recruiter and bomb maker who authorities have linked to the Paris attacks. Laachraoui, who authorities say traveled to Syria in 2013, was linked by DNA to two explosives belts found after the Paris attacks.
A fourth suspect in the plot is being sought by authorities after he abandoned a bomb at the airport and fled the scene.
Conflicting reports from Europe identified Laachraoui alternatively as the third suicide bomber killed in a blast at the airport, and as the still-unidentified man who fled the airport.
"The two deceased terrorists had heavy criminal records unrelated to terrorism," Belgian Federal Prosecutor Frederic Van Leeuw told reporters in Brussels Wednesday.
ISIS claimed credit for the attack through its news agency.
In Turkey, the government announced Wednesday that one of the bombers, Ibrahim Bakraoui, was detained in June and deported to the Netherlands, Agence France Presse reported.
According to the report, the deadly blasts were carried out four days after the arrest of Saleh Abdelslam, one of the organizers of the Paris terror attacks who has been linked to ISIS and its recruitment network in Europe. The Nov. 15 attacks in Paris left 130 people dead.
"While no links between Abdeslam and the March 22 bombings were confirmed, it is evident that the bombings required resources and planning and likely were being prepared for some time," the report said, adding that if Abdeslam was involved in the planning of the bombings, "the timing of the plot may have been pushed up by his arrest, possibly over fears of a disruption."
The bombings in Brussels took place after several recent counterterrorism raids by security forces and shows "the pervasiveness of the network behind the terrorism threat in Belgium and elsewhere in Western Europe," the report said.
A fourth suspect in the terror network was identified as Mohamed Abrini, who was photographed with Abdeslam at a gas station shortly before the Paris attacks.
"In addition to Abrini, Laachraoui, and the suspect in the airport bombing, dozens of other returned foreign fighters and homegrown extremists may currently make up a network of extremists in Belgium and elsewhere in Europe, which is difficult for authorities to map out, let alone to eradicate," the report said.
According to the report, out of the approximately 500 Belgian Islamists who recently traveled to Syria and Iraq, 128 have returned home.
"An estimated 215 Belgian foreign fighters originated from Brussels," the report said.
Most of the Belgian ISIS terrorists come from areas outside Brussels, including about 100 from Antwerp and about 40 from the Flemish Brabant region, including Vilvoorde.
The large number of locations that produced ISIS members suggests that "communities vulnerable to radicalization exist throughout the country," the report said.
Belgian military forces deployed an additional 225 troops after the attacks and boosted security at nuclear power facilities.
Shortly after Tuesday’s attacks, security forces raided a residence in the Schaerbeek area of Brussels that was linked to the airport bombers. Inside, explosives, chemicals, and a black IS flag were found.
The explosive material likely used in the attack was identified as a homemade substance called TATP, for triacetone triperoxide. Thirty-three pounds of the material was found in the Schaerbeek residence, according to news reports.
The report warned that additional police raids are likely and that there is still a risk of additional attacks or copycat strikes.
Despite the attacks, which disrupted air and subway travel, stores and businesses remained opened and the airport is expected to reopen on Saturday.
Cell phone service was temporarily disrupted shortly after the attacks by the high volume of calls, not by any damage to communications networks.
Social media was used as an alternative communications method, and Facebook activated its safety check feature that helped travelers notify others of their status and location.
The State Department report said U.S. private sector companies were not specifically targeted in the bombings, but that the attacks impacted several U.S. firms.
The Stuttgart, Germany-based headquarters of the U.S. European Command restricted travel to Brussels by troops and their families after the bombing.
Brussels is the headquarters of the NATO alliance.