Reports that American Faisal Shahzad flew to his birth country of Pakistan to train for the attempted car-bombing in Times Square highlight that nation's long history as a haven and training ground for terrorists, experts say.

And Shahzad, who is said to have taken four phone calls from someone in Pakistan before buying the Nissan Pathfinder used in the attack, joins a growing string of U.S. citizens or permanent residents who were allegedly groomed and trained by terror leaders there.

They include Najibullah Zazi, who pleaded guilty to plotting to bomb the New York subways; five Washington, D.C.-area men arrested by Pakistani authorities; and Patchogue resident Bryant Neal Vinas, a convert to Islam who went to Pakistan to join al-Qaida.

But however disturbing the recruitment trend, experts also consider it a strengthening sign of U.S. success in keeping more experienced and skillful foreign terrorists out of this country.

"Our enemies . . . have found a path of less resistance by appealing to Islamist sympathizers who are in and can move around the U.S. legally," said Chris Falkenberg, a former Secret Service special agent who runs a corporate security business.

"They can spend as much time as needed traveling abroad and coming back, and they largely can't be stopped at the borders," he said.

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Ramzi Yousef, plotter of the first World Trade Center attack in 1993, and Sept. 11 mastermind Khalid Sheikh Mohammed found refuge for a while in nuclear-armed Pakistan, whose deteriorating internal stability and lawless border region have made it a top national security threat to the United States.

Osama bin Laden himself is believed to be hiding along the border. In recent years, new militant groups with various aims have proliferated, some of them with the tacit support of elements of the government. The public has been torn between resentment of U.S. pressure to rout the Taliban and al-Qaida and anger over terrorist suicide bombings unleashed on their own cities.

"We cannot tolerate a safe haven for terrorists whose location is known and whose intentions are clear," President Barack Obama insisted in a speech last year.

In March, American citizen David Headley, who was born in Washington, D.C., but raised in Pakistan by his father, pleaded guilty to plotting the terrorist attacks in Mumbai, India, that killed 162 people. He had trained at terror camps in Pakistan.

In January 2009, Vinas, 26, was convicted of conspiring in terrorist plots against the United States after joining al-Qaida and training at a camp in Waziristan, Pakistan - the same rugged region where Shahzad reportedly said he trained.

Still, Rep. Peter King (R-Seaford) says the nature of Shahzad's car bomb, which didn't work, is a sign that prolonged U.S.-Pakistani military action along that border has had some success.

"These homegrown terrorists do not have the extensive training that the al-Qaida terrorists had in the late 1990s - now even the training camps they go to are camps on the run," King said.