Some clinics have sponsored foreign patients only to have them fail to show up at their facilities.

The Caster Eye Center in Beverly Hills, Calif., for example, stopped granting such foreign requests after a couple of no-shows.

"In the last few years, we have granted this request only twice. The first was for someone in Uganda, and the other was for someone in Sri Lanka," said Diane Sylvester, surgery coordinator at the Caster Eye Center, one of the leading Lasik eye surgery clinics in Los Angeles. "On both occasions, we issued the letter of invitation, and on both occasions the patient in question never showed up at our facility."

Sylvester told WND the clinic recently has received additional requests for letters from nationals in Pakistan and other al-Qaida hotbeds. Foreign nationals can use the letters to obtain B-2 visitors visas from the State Department to receive medical treatment.

Requests sent to the Caster Eye clinic via e-mail, copies of which were obtained by WND, show nationals have not only requested letters for themselves but for groups as large as a dozen people.

"My concern is that our facility is helping people we cannot personally vouch for to gain entry into the U.S. – or even worse, helping people get visas which are then given or sold into the wrong hands," Sylvester said.

"How many other medical facilities are churning out letters like this under similar circumstances?" she added.

A spokesperson for the State Department, which grants U.S. visas through its embassies abroad, said there are no post-9/11 restrictions on medical facilities issuing invitation letters to foreign nationals. Nor has the department issued any cautions to the health-care industry.

"I'm not sure which I'm more alarmed by – people scamming for visas, or the casual attitude of those overseeing the granting of visas," Sylvester said.

The department added, however, that a letter of invitation from a medical facility does not necessarily guarantee approval of a foreign patient's visa.

In the wake of the recent "doctor jihad" in the UK, the FBI and Department of Homeland Security are scrutinizing foreign nationals who have applied to the U.S. for visas to attend medical school or practice medicine here.

Two of the UK physicians who plotted to car-bomb London's entertainment district had applied for permission to work in the U.S. One made contact with the Philadelphia-based Educational Commission for Foreign Medical Graduates.

Terrorists posing as patients also are a growing concern, federal authorities say.