An application for Bitcoin in front of a window of the offices of the bank 'La Maison du Bitcoin' in Paris, France.

Two alleged thieves targeted cryptocurrency executives and threatened their families, stealing hundreds of thousands of dollars in a wide-ranging scheme, according to a Justice Department indictment handed down Thursday in Massachusetts.

Eric Meiggs and Declan Harrington, both of Massachusetts, allegedly targeted cryptocurrency executives and commentators, including individuals who published crypocurrency trading guidance, owners of blockchain-based businesses and cryptocurrency project leaders. In all, they were allegedly stole or attempted to steal $550,000.

The indictments highlight fears that scammers will target highly public participants in the still-young cryptocurrency space. The FBI has issued warnings to early cryptocurrency adopters to be on alert and protect their private information.

The complaint says Meiggs and Harrington used cell phone SIM card swapping to gain access to victims' cryptocoin accounts, and sent hostile messages to targets, often threatening their families.

SIM swapping exploits the process through which cell phone carriers assign phone numbers to new phones. Thieves convince the victim's cell carrier to reassign their number to a phone controlled by the criminal. They then use the phone to authenticate monetary transactions.

In one case, the defendants allegedly used the mobile device access to gain further access to a California-based victim's Gmail and Yahoo email accounts, which contained tax returns and passwords. "They then changed the passwords on [the victim's] online accounts," the indictment reads. The complaint says they were able to steal $10,000 from this victim, and also accessed his Linkedin, Facebook and Twitter accounts. They even sent a text message to the victim's daughter, saying "Tell your dad to give us bitcoin," according to the complaint.

In another case, Meiggs allegedly harrassed a Michigan-based victim, sending the victim messages showing that he knew his and his mother's addresses, and threatening to kill the victim's wife if he did not give up his Instagram handle.

The pair along with co-conspirators allegedly stole $165,000 from another victim by gaining access to his Gmail account and finding a private key there for a cryptocurrency wallet.

Follow @CNBCtech on Twitter for the latest tech industry news.