LOWELL -- After making three arrests, police are searching for a fourth suspect in a fentanyl trafficking investigation, according to a press release.

Kevin Davidson, 33, of Lowell is wanted in relation to the investigation and on outstanding warrants, police said. He was last seen driving a white pick-up truck with New Hampshire plates.

Anyone with information regarding his whereabouts can contact the Lowell Police Department at 978-937-3200.

Police arrested three others on charges related to the investigation on Wednesday, according to the release.

Luis Rosario, 28, of Lowell, was charged with trafficking fentanyl over 10 grams. Ryan Kelly, 28, of Stoneham, was charged with possession of a Class A substance.

Patrick Riley, 45, of Lowell, was charged with possession of a Class A substance and possession of a Class E substance.

Riley later died after being released from police custody following a court appearance, according to police. Capt. James Hodgdon declined to confirm the cause of death, citing medical privacy laws.

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