An 18-year-old freshman has been missing for more than 11 days since unexpectedly boarding a plane for London, Northeastern authorities said.

William Lamar withdrew money from his savings account and purchased an American Airlines ticket to London Oct. 28, said Ana Hebra Flaster, a longtime family friend and neighbor. He purchased a return ticket home for Nov. 4, but never boarded that flight, Flaster said.

Lamar has Asperger’s syndrome, a form of autism marked by difficulty interacting with others socially, said Olga Guttag, another longtime friend who is speaking for the family.

After Lamar failed to return home Friday evening, Interpol and the Federal Bureau of Investigation (FBI) were contacted, and are now working with Boston Police, Scotland Yard and Northeastern Police to locate the student, said James Ferrier, associate director of public safety.

“We had made preliminary contact with Interpol through the FBI that if he did not return on the flight Friday that we would send out the [Interpol] alert,” Ferrier said.

Flyers containing information about Lamar’s height, weight and where he was last seen have been distributed to various American embassies throughout Europe so authorities can keep a lookout for him, Ferrier said.

Lamar’s family reported even though Lamar has run away from home before, he has never gone missing for longer than a few hours, Ferrier said. Police at this time do not believe any foul play is involved with his disappearance, Ferrier said.

Asperger’s syndrome is often known as a “high-functioning” form of autism, said Lori Normandin, a licensed Massachusetts social worker who works with children with the condition. Most with Asperger’s syndrome are highly intelligent with advanced vocabularies, but have trouble socializing with others, she said.

“They don’t understand social cues,” she said. “They really don’t understand how to get along socially with others. They don’t have good eye contact usually, and they don’t understand when to start and stop conversations.”

Normandin said Lamar most likely has a less advanced form of the condition because of his ability to live on his own at Northeastern and travel overseas on his own. Flaster confirmed this, and described Lamar as “very, very high- functioning.”

She also said Lamar is extremely interested in computers and loves reading and spending time in libraries. He enjoys languages, taught himself German and is an expert on Esperanto, the international language created in 1887 as a means to end the worldwide language barrier.

Many with Asperger’s Syndrome develop advanced interests in certain areas, Normandin said. Lamar is no different, and has an exceptional knowledge of Germany, Ferrier said, besides his fluency in the language. Lamar’s passion has led police to focus their search on Germany as well as England, Ferrier said.

So far, Lamar has yet to use his ATM card since he purchased his airline tickets, Ferrier said. Police have monitored the trains in Europe to make sure Lamar has not traveled into Germany.

There has been no activity on his Northeastern e-mail account, and police are currently working to get a court order to view his off-campus e-mail accounts to see if he has attempted to communicate with anyone through the Internet, Ferrier said.

As authorities continue to search for Lamar, Guttag said she is grateful for the cooperation she has received so far from Northeastern.

“Northeastern, all the way from the president down to the dorms, has been unbelievably helpful in this,” she said. “We have gotten nothing but incredible cooperation from Northeastern. That has been the light in the situation.”

Lamar’s family said they want to make sure he knows he isn’t in trouble; they simply want to know he is safe. Anyone who was on American Airlines Flight 156 is requested to contact Northeastern Public Safety.

A fund has also been set up by family and friends to help the family with the expenses of an international search, since the family is already dealing with mounting expenses due to his father’s struggle with Parkinson’s Disease.

The William Lamar and Family Help Fund is set up through Sovereign Bank in Lexington. The contact person for this fund is Neerja Kalloo. She can be reached at (781) 676-0301.