Suffolk police and federal agents are investigating whether the fatal shooting of a Bay Shore woman and her unborn child was a revenge killing by MS-13 gang members who suspected she was an informant, according to sources familiar with the case.

Revenge as a motive for the Saturday night killing of Milagro G. Canjura, 31, is the strongest theory among several being pursued by investigators who have delved into her personal life in the hunt for clues, the sources said.

Investigators with the Department of Homeland Security and Suffolk homicide have focused on Canjura's relationship with an MS-13 gang member from Queens who was indicted in a February gang-related killing two days before she was shot to death, sources said.

Her death served as a reminder that MS-13 gang members on Long Island won't hesitate to kill mothers and children in a deadly pursuit of revenge.

MS-13, also known as Mara Salvatrucha, is an extremely violent transnational street gang that federal officials have identified as a national security threat. Suffolk police at one point targeted MS-13 as part of a federal task force before department leaders pulled their detectives from the unit in 2012 in favor of a precinct-based approach.

In 2010, Vanessa Argueta and her 2-year-old son, Diego Torres, were shot to death in a Central Islip vacant lot by MS-13 gang members.

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Canjura, who neighbors said was expecting a daughter in December, was an acquaintance of MS-13 member Byron Lopez, 23, the suspect in the February killing, sources said. Lopez occasionally lived with Canjura in Bay Shore, according to the sources.

As federal investigators homed in on Lopez as the prime suspect in the shooting of MS-13 gang member Sidney Valverde, they questioned Canjura about his gang activities, the sources said.

Federal prosecutors said that on Feb. 25 Lopez lured Valverde to Long Island by telling him he was needed to assist in gang activities. Instead, Lopez shot Valverde in the back of the head and dumped his body on a beach in Miller Place, prosecutors said. MS-13 wanted Valverde dead because they suspected he was also an informant, federal prosecutors said.

The body of Valverde was discovered on March 12 in front of the Miller Beach Surf Club, police said.

In court filings, prosecutors did not identify who cooperated as they built the case against Lopez. Evidence against him included witness testimony, cellphone records and admissions by Lopez in his Facebook account, according to court records. Prosecutors declined to give details of the admissions. Lopez was indicted in Valverde's killing on Aug. 28 and pleaded not guilty. He is being held without bail.

Two days after the Lopez indictment, Canjura arrived at her Stein Street home, picked up her mail and walked back to her car, according to a witness who did want to be identified. Six gunshots rang out at about 8:30 p.m., the neighbor said. The car's windows shattered and Canjura, who planned to name her daughter Kamila, according to a neighbor, was inside. She was bleeding from the head, neck and arms after being hit by multiple gunshots, a neighbor said. When police arrived, she was unresponsive.

Canjura and her unborn child were pronounced dead at Southside Hospital, police said.