MARTINEZ, Calif. (KRON) ? Authorities believe four homicide cold cases in Contra Costa County are connected to gang activity.

The Contra Costa County District Attorney?s Office said it filed three cases for four homicide cold cases; one of which happened in 2014 and the others in 2015. There is a total of 11 defendants, allegedly members of the Sure?os gang.

?This successful operation started with a wiretap and led to multiple gang members involved in senseless murders and violence being arrested,? District Attorney Diana Becton said in a press release Tuesday. ?While these cases were not solved right away, Concord Police and the FBI Safe Streets Task Force did not give up and fortunately we can bring some closure to the victims? families.?

The DA?s Office identified the victims as follows:

Victor Gutierrez, allegedly killed on April 17, 2014
Erick Cruz, allegedly killed on Sept. 12, 2015
Marcos Villazon, allegedly killed on Nov. 21, 2015
Luis Estrada, allegedly killed on Nov. 30, 2015
Cases were filed against the following suspects:

People v. Michael Valdez, Andrew Cervantes, Daniel Rodriguez
People v. Rafael Lopez & Juan Barocio Jr.
People v. Jose Cisneros, Marcos Ochoa, Luis Cruz, Aurelia Mendez, Antonio Mendez, Jose Ochoa
A total of 31 people were arrested in this investigation, called Operation Boulevard Blues. According to the DA?s Office, ?34 search warrants were executed in multiple locations across Contra Costa County and 42 firearms were recovered.?

The investigation is still active.