Three laptops and a cellphone have been stolen since September from students who left them unattended while studying in Boston University’s main library, and police believe one suspect is responsible for all four thefts.

A surveillance camera image of the suspect (BU Police Department)

“There is a person who [police] think has been in there more than once,” said Colin Riley, a BU spokesman. “Each time he dresses differently and has changed his appearance slightly.”

The items were stolen at the Mugar Memorial Library at 771 Commonwealth Ave. since Sept. 30, officials said.

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“When people go to the men’s room or ladies’ room, rather than pack all their gear with them, they walk away from it,” Riley said. “It only takes a few seconds for it disappear.”

In 85 percent of all thefts at the university, the valuables were unattended when stolen, BU Police Detective Lieutenant Peter DiDomenica said.

The first theft occurred Sept. 30, the second Nov. 1, and the third on Nov. 17, DiDomenica said. In each incident, the suspect spent less than 10 minutes in the library. In one case, both a laptop and a cellphone were grabbed at the same time.

“The times vary throughout the afternoon, and one was at seven in the evening,” DiDomenica said. “There is no discernible pattern.”

The suspect is college-aged, but police are unsure if he is a BU student.

Laptop thefts are down 35 percent from last year at BU. Between September 2012 and mid-November, there were 26 laptop thefts, DiDomenica said. There have been 17 thefts in the same period this year.

“This is disturbing, but overall we are improving in terms of the number of thefts,” DiDomenica said.

Police said they would increase their uniform and plainclothes presence in the library and throughout campus in wake of the thefts.