BOSTON (AP) — The U.S. Centers for Disease Control and Prevention will be helping Massachusetts public health officials investigate a recent spike in HIV infections in Lowell and Lawrence.

Authorities said Thursday that a cluster of new HIV infections has occurred largely among the homeless and people who inject drugs.

The state Department of Public Health requested help from the CDC after at least 52 new HIV cases were reported among drug users in northeastern Massachusetts in 2017, compared to 23 the previous year.

State officials are puzzled as to why the increase is happening now, after rates of HIV transmission had remained stable for several years despite growing use of injection drugs tied to opioid addiction.

The CDC is expected to offer assistance in several areas, including interviews with recently-infected individuals.