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Alexander Mercado, a 20-year-old Chicago man, was arrested on Thursday and charged with obstruction of justice.
Mercado pleaded not guilty during his initial appearance and arraignment in federal court Friday, where he wiped away tears after entering his plea.
He allegedly deleted the Signal app on his phone after being contacted by an FBI agent on June 13, one day before the UFC event, the DOJ said.
According to an indictment, the agent asked Mercado in a telephone call whether he planned to travel to Washington, D.C. to help with the attack, which Mercado denied.
Mercado later allegedly did not respond to follow-up calls attempting to reach him, and investigators found he uninstalled the Signal app on his phone, which led all of his data related to his messages in the group being deleted.
Seven other people have been charged in connection with the plan.
Prosecutors on Friday asked that Mercado remain in custody, arguing he was the "administrator" of the group, engaged in recruitment efforts, contacted a high-ranking member who remains at large, and poses risks of obstruction of justice and flight. They also argued Mercado has an unstable living situation, noting he recently moved and lives with his mother, who was the alleged victim in a domestic battery case in October 2025 involving Mercado that was later dismissed.
The defense countered that the family's recent move does not make his housing unstable, noted the domestic case was dropped shortly after it was filed, and argued there is no evidence Mercado knows the identities of other group members because the platform is anonymous. The judge said he does not consider Mercado a flight risk based on his housing situation alone but found the government met the legal standard to hold a detention hearing, which is set for Tuesday.
Prosecutors also moved to unseal the indictment, and the case has been assigned to U.S. District Judge John Blakey.
Mercado faces up to 20 years in prison, if convicted.
An attorney for Mercado could not immediately be reached as of Friday afternoon.
ABC News contributed to this report.