Would-be ISIS terrorists are taking more roundabout routes to Syria in an effort to make European authorities' attempts to catch them more difficult.

Western officials and diplomats tell the Wall Street Journal that many foreign fighters still try to fly directly to Turkey, where they can slip over the border to join ISIS, which has its main stronghold in northeastern Syria. 

However, they say an increasing number are taking longer, indirect trips that make it easier for them to hide their journey's true nature. Most notably, Hayat Boumeddiene, the wife of Paris kosher grocery gunman Ahmed Coulibaly, traveled to Syria last month via Turkey and Madrid, Spain. Others travel to visit families or to vacation destinations in the hope of throwing counterterror monitors off their scent. The phrase officials use to describe this phenomenon is "broken travel."

At least 3,400 European fighters are believed to have joined the ISIS army, which is estimated to be approximately 20,000 strong. That number dwarfs the 100 or so fighters believed to come from the U.S.

The practice of "broken travel" is a challenge for European Union officials, who have long been committed to the free movement of people between E.U. member nations. As a result, border checkpoints and passport checks for European citizens have been essentially abolished in practice. 

This past week, E.U. countries have agreed to share more information with each other and expand the continent's law enforcement database to block potential fighters from traveling. However, European officials tell the Journal that there remain border crossings that are not adequately patrolled or protected by intelligence agencies. 

Among those sites are the border between Turkey and Bulgaria, which stretches 170 miles, while another is the ferries between northern Cyprus, controlled by Turkey, and that country's mainland. Cyprus is a popular destination with tourists, with authorities estimating that approximately 2 million people visit every year. A Cypriot security official tells the Journal that a dozen militants have used the ferry route to travel to Turkey and on to join ISIS. 

It is not clear how many people have crossed the Bulgaria-Turkey border en route to Syria, but the country's interior minister put the number in the hundreds last month. The Journal reports that U.S. troops began training Bulgarian border police last year. More recently, the government in Sofia approved expansion of a fence along the Turkey border.

However, attempts at cooperation between law enforcement officials from the two countries have been hampered by mutual accusations of laxity. For example, Bulgarian authorities say that Turkish officials aren't doing enough to prevent would-be terrorists from returning to Europe. For their part, Turkey says the Bulgarian police have routinely missed suspects they should have picked up. 

For example, days after the deadly attack on the satirical magazine Charlie Hebdo in Paris, Bulgarian police arrested Fritz-Joly Joachin, a self-confessed associate of the attackers. However, Turkish officials say Joachin was traveling to Syria with three other people who were allowed to continue toward the border. The three were later arrested by Turkish police and extradited to France. 

"The Bulgarians missed these people, and they shouldn't have," a senior Turkish foreign ministry official told the Journal. "It's not the first time this has happened, and it's a problem."

Click for more from The Wall Street Journal.