Boston police today said they no longer believe a woman who claimed on Aug. 18 that she was sexually assaulted in Allston by a man who was driving a dark sedan, one of two incidents that raised concerns about people getting rides from unlicensed taxis or livery cars.

In a brief statement posted on their website this afternoon, Boston police said the sexual assault investigation that was started after the woman contacted police is now closed.

“Evidence, witness interviews and statements, led detectives to believe the facts of the case did not support the allegation as reported,’’ police said in the statement.

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Police had previously said the woman told them she tried to hail a taxi near the intersection of Linden and Pratt streets around 2 a.m. on Aug. 18. A man driving a dark, four-door, mid-sized sedan stopped and offered to drive her to Cambridge, police said.

The female told officers she was taken to an area she believed was near Harvard University’s campus along North Harvard Street in Allston near the Boston neighborhood’s border with Cambridge, where the driver sexually assaulted her.

The Allston incident was the second police had connected to unlicensed vehicles for hire. The first took place Aug. 6 when a woman in the Seaport District signaled for a taxi near Seaport Boulevard and Northern Avenue and was picked up a black Lincoln livery sedan, officials have said.

The woman was driven to a secluded area on state property in front of the Daly Memorial Ice Skating Rink off Nonantum Road, near the border of Newton and Brighton, where she was sexually assaulted.

Police continue to search for the suspect in that case.