A federal grand jury in San Jose indicted 12 alleged members of a violent Santa Cruz-based street gang on charges including murder, arson, extortion and racketeering conspiracy on Thursday.

The indictment of the alleged Mara Salvatrucha gang members comes months after the 12 suspects were originally charged in April. The charges reflect new allegations against some defendants who are suspected of conspiring to commit, attempting to commit, and succeeding in committing murder, according to the U.S. Attorney’s Northern District of California office.

The amended indictment alleges that six alleged gang members — Velarmino Escobar-Ayala, 38, Tomas Rivera, 25, Melvin Lopez, 26, Alexander Martinez-Flores, 29, Erik Escalante-Torres, 23, and Jose Noe Ramirez-Avelar, 28 — conspired to commit a murder, and that all of the men, except for Lopez, committed the murder.

Escalante-Torres, 23, is also accused of attempting to commit the murder of another victim, officials said Thursday.

Also indicted were Ismael Alvarenga-Rivera, Willfredo Ayala-Garcia, Jose David Abrego-Galdamez, Gerber Morales, Emilio Escobar-Albarnga and Josue Alcedis Escobar-Cerritos. All 12 were accused of racketeering and drug trafficking.

The indictment also alleges that some defendants conspired to extort property from drug dealers in Santa Cruz by threatening violence.

The amended indictment comes after a multi-year investigation into the activities of a Santa Cruz gang known as the Santa Cruz Salvatrucha Locos 13, which is believed to be a subset of MS-13, officials said.

Lauren Hernandez is a San Francisco Chronicle staff writer. Email: lauren.hernandez@sfchronicle.com Twitter: @LaurenPorFavor