Despite the high death toll and dramatic scenes of destruction, this week's attacks in Brussels appear to have been surprisingly easy to carry out, requiring little more than some careful preparation, a handful of motivated militants and ingredients that are readily available on store shelves.

Security experts say Europe's major cities — filled with soft targets and home to hundreds of Islamic militants who have fought or trained in Syria, Iraq and Libya — will remain vulnerable to similar attacks without changes in their security procedures.

The assailants in Brussels were well-prepared for the suicide bombings in the airport and subway, which killed more than 30 people. They chose crowded, easy-to-reach targets that were poorly secured in a country whose forces have already been stretched by a string of crackdowns on suspected Islamic militants. Belgium's chief prosecutor said the investigators found 15 kilograms of TATP — an inexpensive and hard-to-detect explosive material — at an apartment where the attackers had stayed, but it wasn't immediately clear whether it was used in the blasts.

"It doesn't require sophistication, but it requires preparation and planning," said Yoram Schweitzer, a former head of the Israeli military's counter-terrorism desk. "There is a need for suicide belts, a safe house, or perhaps baggage with explosives."

Schweitzer, an expert at the Institute for National Security Studies, an Israeli think tank, estimated that an attack like the one in Brussels would take weeks, perhaps several months, to plan.

Israel faced a wave of suicide bombings during the second Palestinian uprising in the early 2000s before taking a series of measures that halted the attacks. These included construction of a massive separation barrier to block attackers from the West Bank, a military crackdown and stepped up intelligence, including the use of Palestinian informants, that allows Israel to nab suspects before they carry out their operations.

Equally critically, Palestinian President Mahmoud Abbas, who took office in late 2004, has maintained a system of security cooperation with Israel, even during times of heightened tensions. This cooperation has been motivated by shared concerns over the Islamic militant Hamas group.

Israel's Ben-Gurion Airport has not experienced an attack in decades, thanks to a sophisticated multilayered system of security checks that include inspections of every vehicle entering the site and armed guards both inside and outside the terminals. Schools, supermarkets and shopping malls all have security guards who check visitors' bags. Racial profiling is common, and Arab travelers and visitors are often frisked or aggressively questioned.

While Israelis have become accustomed to these inconveniences, bringing such measures to continental Europe, with its larger territory and diverse population, would be difficult.

Citizens can pass freely across European borders without stringent identity checks, and transport hubs contain very few security checks. Profiling suspects is challenging in Europe due to its racial diversity, and extremist groups like IS actively recruit people of European descent.

Armed guards patrol many of Europe's train stations, airports and landmarks, but their presence is significantly less onerous than similar hubs in the U.S. and Israel. There is also direct train access with little to no security into many European airports. The Belgium attackers appear to have entered the Brussels airport posing as travelers with suitcases.

"Getting a bomb on a plane or through the security at the airport is tough, but getting one into the airport or a train station is relatively simple," said Matthew Henman, managing editor at IHS Jane's Terrorism and Insurgency Centre in the U.K. "But imagine if you had everyone's bags searched before they entered at train stations during peak rush-hours. Soon, there would be long lines and those gathered crowds would become the new target."

The United Kingdom is better protected because of its physical separation from continental Europe, allowing for better border controls and greater difficulty smuggling weapons into the country.

Still, homegrown suicide bombers struck London in simultaneous attacks that killed 56 people in 2005. Since then, Britain has invested in placing protective barriers around buildings, improved communications systems in the London subway system and an extensive network of security cameras around transport hubs and landmarks.

Recently, British counter-terrorism teams have been planning for an even scarier scenario: a chemical or biological attack.

"Groups like IS learn on their feet," Henman said. "Once one set of additional measures are introduced, they'll learn from the changes and will soon alter their operations."

One of the biggest challenges for European authorities will be improving intelligence gathering. Security experts estimate hundreds of militants who have fought in Syria are now in Europe, many of them in Brussels. They tend to live in insular immigrant communities that have been difficult for law enforcement agencies to penetrate.

Authorities will have to learn how to foil these attacks as they are being planned, said Elias Hanna, a former Lebanese brigadier general who is a lecturer at the American University of Beirut. "You have to be ahead of them in time and preparation," he said.

On Wednesday, Prime Minister Benjamin Netanyahu offered Israeli intelligence assistance to Belgium.

Shlomo Harnoy, a former senior official at Israel's Shin Bet internal security agency, said that global airport security remains focused on preventing attacks on airplanes, a remnant of the 9/11 attacks, while ignoring security inside and outside terminals.

It was a 2006 trans-Atlantic bomb plot which introduced bans on liquids being carried through airport security and onto planes. He said authorities must do a better job detecting bombers, not just bombs.

"When you are busy taking a bottle of mineral water away from an old woman, you miss the big picture," said Harnoy, a founder of Sdema Group, a homeland security consulting firm. "They are too busy with the routine checks instead of scanning for suspects."

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Dodds reported from London. AP correspondents Aron Heller and Ian Deitch in Jerusalem, and Philip Issa in Beirut contributed to this report.