The only person criminally charged in connection with the 2015 San Bernardino terror attack pleaded guilty Thursday to providing the high-powered rifles that were used to kill 14 people.

Enrique Marquez Jr., 25, appeared in federal court in Riverside, Calif., after signing a plea agreement with prosecutors.

He was accused of providing husband-and-wife attackers Syed Rizwan Farook and Tashfeen Malik the rifles used in the rampage, as well as plotting mass killings with Farook several years earlier that were never carried out.

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During Thursday’s hearing, U.S. District Judge Jesus Bernal allowed Gregory Clayborn, whose daughter, Sierra Clayborn, was killed in the attack, to address federal prosecutors. He expressed his disappointment in them for the plea agreement saying that Marquez was getting “a slap on the wrist.”

He added, “People get more time for committing a lesser crime.”

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Bernal said the victims' families would be able to speak before Marquez is sentenced.

Under the plea deal, Marquez could face up to 25 years in prison. A sentencing hearing is scheduled for Aug. 21.

The 2015 attack at a holiday party for county health workers also wounded 22 people. Farook and Malik were killed in a gunfight with law enforcement later that day.

Prosecutors said there was no evidence Marquez, a friend of Farook's, participated in the San Bernardino massacre or had advance knowledge of it.

Prosecutors said Marquez acknowledged being a "straw buyer" when he purchased two AR-15 rifles from a sporting goods store that were used in Farook's attack on the meeting of his work colleagues.

Prosecutors have said Marquez agreed to buy the weapons because the attackers feared Farook's Middle Eastern appearance might arouse suspicion.

Marquez also acknowledged plotting with Farook in 2011 and 2012 to massacre college students and gun down motorists on a gridlocked Southern California freeway, though those attacks never occurred.

Marquez said he backed out of the plot after four men in the area were arrested on terrorism charges in late 2012, the FBI has said in court documents.

Marquez and Farook met in 2005 after Marquez moved next door to Farook's family in Riverside, about 55 miles east of Los Angeles.

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Farook began educating his new friend about his Muslim faith and Marquez converted in 2007.

The FBI said the two began discussing extremist views shortly thereafter. By late 2011, Marquez spent time at Farook's home reading magazines published by Al Qaeda and studying radical material online, federal officials said.

Fox News' Dan Gallo in Riverside and The Associated Press contributed to this report.