Two individuals were taken into custody in Massachusetts, with a third arrested in Maine.

One focus of the probe, according to a source close to the investigation, is a system of "cash couriers" who bring money into the United States from overseas, a method thought to help finance operations like the attempted Times Square attack.

A statement from US Immigration and Customs Enforcement confirmed that three people were taken into custody on alleged immigration violations, but provided no further details.

Two individuals have been under surveillance at least since Wednesday in connection with a cash courier system involving the Times Square bombing attempt, a source close to the investigation said, but the source could not confirm that they were two of the three individuals taken into custody on Thursday.

Two of those arrested on Thursday are so-called "visa overstays." Another was in the process of being removed from the United States, but had not yet been ordered removed, as he was attempting to adjust his status so that he could stay in the country, the official said.

Attorney General Eric Holder said on Thursday that those arrested are tied to Faisal Shahzad, the Pakistani American charged in the May 1 attempted bombing at Times Square, though the exact nature of the relationship is unclear.