The Amherst Democrat was among several senators absent during Wednesday's largely ceremonial session to begin the new year. Aides say Rosenberg, who is working out of a temporary basement office in the Statehouse for the duration of the investigation, plans to attend formal sessions going forward.
The Senate Ethics Committee hired a Boston law firm last month to look into whether he violated any Senate rules in connection with allegations reported by The Boston Globe that his husband, Bryon Hefner, sexually assaulted several men, including some with business before the Legislature.
Rosenberg has not been accused of any wrongdoing.
Acting Senate President Harriette Chandler, a Worcester Democrat, made no reference to the probe during opening remarks to the chamber.
She listed affordable housing, health care, criminal justice reform and the minimum wage among her priorities for the coming months, adding that lawmakers may also address "overreaches of the federal government that would negatively impact us, including to investigate and mitigate the consequences of the net neutrality overreach of the FCC, and of the federal tax bill."
Republican Gov. Charlie Baker delivered the ceremonial oath of office to newly elected Sen. Dean Tran, a Fitchburg Republican who is the first Vietnamese-born member of the Legislature. He was a young child when his family fled war-torn Saigon.
Tran won a special election to succeed Democrat Jennifer Flanagan, who resigned from the Senate following her appointment to the state's Cannabis Control Commission.
The House also held its first meeting of the new year on Wednesday, dealing with mostly routine matters.
The Legislature is scheduled to hold formal sessions through July 31.
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