A bearded bank robber with a shaved head likely struck for the third time in less than a week by holding up a Farmingdale credit union Thursday morning, Suffolk police said.

He entered the Bethpage Federal Credit Union on Route 109 at 9:30 a.m. and ordered a teller to give him money, handing the employee a demand note, said Det. Sgt. Edward Compagnone of the pattern crime unit, which has arrested several bank robbery suspects this year amid a spike in such crimes.

The robber escaped on foot, police said.

He appears to be the same man who fled with cash from a Capital One branch on Commack Road in Deer Park on Saturday and the Bank of America on Veterans Highway in Commack on Tuesday, Compagnone said.

Compagnone said video surveillance from the three financial institutions and witness statements indicate the police have another serial robber on their hands.

The suspect was described as between 30 and 35 years old, with a shaved head, a thin beard and medium build, police said. He was about 5 feet, 10 inches tall, and wore jeans and a T-shirt, police said.

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The spate of Long Island bank robberies this year has been unusual, police said. There were 21 cases from Jan. 1 to Jan. 29, police have said. Twelve of those were in Suffolk, compared with 19 bank robberies for all of 2013, police said.

The bank robberies have continued despite arrests of accused serial bank robbers in Nassau and Suffolk counties.

No one overriding reason is seen for the spike in bank robberies this year, Compagnone said. Such robberies may be easy to pull off, he said, but forensic evidence and public tips have led to a "good success rate" in nabbing suspects.

Detectives said anyone with information can call Crime Stoppers at 800-220-8477.