An immigration lawyer whose cellphone was seized by federal agents at Boston Logan International Airport last month knew the Department of Homeland Security was investigating hiring practices at his law firm when agents asked for his phone, according to court documents filed by prosecutors Tuesday.

Federal agents had “reasonable suspicion,” when they seized Andrew G. Lattarulo’s business phone on Sept. 28 during a lawful border search, that the law firm where Lattarulo is a founding partner, Georges Cotes Law, had employed “aliens,” unauthorized to work in the country, prosecutors said in court records.

The investigation is ongoing, and civil and criminal penalties are a possibility, according to the filing in US District Court in Boston.

Lattarulo has a law office in Chelsea and discus

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