Target: Syed Rizwan Farook, 29, converted Enrique Marquez Jr, 25, into a fanatical Muslim then used him to help prepare for the massacre

The father of one of the San Bernardino terror attack victims has called the fact that the Muslim convert who aided the mass murderer will escape a life term a 'travesty'.

Enrique Marquez Jr was in court in Riverside, California, Thursday to enter a guilty plea as part of a deal that will see him sentenced to a maximum of 25 years.

He bought guns which were used in the massacre and had been plotting other terror attacks for years with Syed Rizwan Farook.

He had been facing up to 50 years behind bars if he had been convicted after a trial.

But Gregory Clayborn, 58, of Los Angeles, California, stood up before Judge Jesus G. Bernal to demand he gets life without parole. Outside court, he called the deal a 'travesty'.

Clayborn's daughter Sienna, 27, was one of the 14 gunned down by Farook, 29, and his Pakistani wife Tashfeen Malik, 30, at the Inland Regional Center in San Bernardino on December 2 2015.

Fighting back tears, he said: 'Why is this man [Marquez] not being charged with murder? If he had not purchased those weapons, we would not be here.

He added: He should get life without parole. A whole life. He's going to get a slap on the wrist. I can't get my head around it.

Husband and wife killers: Syed Rizwan Farook, 28, brought his Pakistan-born wife Tashfeen Malik, 29, into the country just over a year before they launched their deadly attack together

Victim: Sierra Clayborn was one of the 14 murdered by the two Islamic fanatics at the Inland Regional Center in San Bernardino in December 2015

'Travesty': Gregory Claybourn, who lost his daughter Sierra, spoke in court to ask why Marquez was not being jailed for life and outside said: 'It's a travesty and the judicial system needs to fix it.'

Not forgotten: The victims of the San Bernardino shooting rampage. They are top row left: Robert Adams, Isaac Amanios, Bennetta Betbadal, Harry Bowman and Sierra Clayborn. Second row from left: Juan Espinoza, Aurora Godoy, Shannon Johnson, Larry Daniel Kaufman and Damian Meins. Bottom row from left: Tin Nguyen, Nicholas Thalasinos, Yvette Velasco and Michael Wetzel.

'Those 22 people who were injured - their lives will never be the same. My life will never be the same. Your Honor, please give us closure.'

Outside court, fighting back tears, he added: 'Why is this man [Marquez] not being charged with murder? If he had not purchased those weapons, we would not be here.

He added: 'He should get life without parole. A whole life. He's going to get a slap on the wrist. I can't get my head around it.'

Speaking outside court after the hearing concluded, Claybourn described the 25-year maximum sentence as 'a travesty' and complained that 'criminals have more rights than we do'.

He added: 'Being surprised like this, with what happened, and this guy not being charged with the actual murders, it just made me angry. It really made me angry.

'It just broke my heart because the thing is, if you've never been in this position, if you've never had your child taken from you by such callous people, you begin to include all folks who are working, seemingly thinking for that purpose.

'Criminals have more rights than we do. They go into the system and they have more rights than we do.

Understanding: Eileen Decker, U.S. Attorney for the Central District of California said she felt for those who wanted a longer sentence but could only jail Marquez on the evidence she had

In court: Enrique Marquez appeared on Thursday at Riverside Federal Court in California. He is expected to be sent to the federal supermax

'Basically, what I said in there [in court], I meant it. For him to get as light a sentence as he got, for the crimes that were committed based on what he provided [weapons], it's a travesty and the judicial system needs to fix it.'

Asked if if he believed Marquez' claims that he knew nothing of Farook's plans, Claybourn replied 'Hell, no.'

Responding to Claybourn's concerns, US Attorney Eileen Decker said the decision to go for the deal had been made based on the evidence available and said the only way Marquez could have been eligible for a whole life term was if he had been charged with murder.

She added: 'My heart goes out to him [Claybourn]. My job is to follow the evidence and the law.

'But I understand why he and some of the other victims' families think 25 years is not enough. I sympathize with their view.'

Decker said a decision on which jail Marquez will ultimately be sent to would not be taken until after sentencing. He is likely to at least initially be sent to the federal supermax in Colorado.

During the hearing, Marquez admitted his guilt on two counts - one of conspiring to provide material support to terrorists and another of making a false statement in connection with the acquisition of a firearm.

Handcuffed and wearing a white prison-issue jumpsuit, he made his answers quietly and told the court that he is seeing a psychologist and taking lithium which is used to treat depression and anxiety.

Judge Bernal accepted Marquez' guilty plea and adjourned sentencing until August 24.

Marquez had been facing up to 50 years in jail but is likely to serve a maximum of 25 followed by a lifetime supervision order and a fine of $500,000.

A third charge of immigration fraud was struck out as part of the deal, which came less than a month before Marquez was scheduled to go to trial.

In court: Enrique Marquez Jr. was previously seen in court in Riverside, California, in December 2015 when he appeared in shackles charged with providing material support to the attackers. He is now due to plead guilty as part of a proposed plea deal

Massive armed presence: Law enforcement officers poured into San Bernardino because of the massacre at the social services center

Marquez was arrested on December 6 2015 three days after his friend Farook massacred 14 with his wife Malik at the Inland Regional Center in San Bernardino.

The couple, who died in a hail of gunfire at the hands of cops the same afternoon, had pledged allegiance to ISIS - although Farook had previously expressed a wish to join al Qaeda.

That night, Marquez posted a cryptic message on Facebook saying that 'it was a pleasure knowing everyone'.

The next day, he consumed nine bottles of beer and checked himself into UCLA Harbor Medical Center in Torrance, California, where staff said he appeared 'emotional' and 'anxious'.

He also called 911 to confess to knowing Farook and added: 'He was the shooter. The f***ing a**hole used my gun in the shooting.'

Marquez, who was driven to court from a maximum security federal prison in downtown Los Angeles, first met Farook in 2005 when he was just 14.

Two years later, he converted to Islam, began attending a mosque in Corona, California, and became fascinated by the teachings of Anwar al Awlaki.

Al Awlaki, a radical American cleric, died in a drone strike in Yemen in 2011 and was a leading light in al Qaeda.

He also helped prepare underwear bomber Umar Farouk Ummuttallab, preached to three of the 9/11 hijackers and corresponded with Fort Hood shooter Major Niswan Hasan in the run-up to the 2009 attack.

With Farook, terrorist's apprentice Marquez plotted attacks on the 91 Freeway and Riverside City College.

American-born: Marquez, a native-born citizen was converted by his citizen neighbor Farook - who used the speeches of another US citizen Anwar al Awlaki to talk him into radical Islam

Aftermath: 14 people died, many more were wounded, and those who survived were escorted to safety when the terrorists hit

Final moments: The two San Bernardino mass murderers died in a hail of bullets as police finally caught the SUV in which they had made their escape

Both plots involved the use of pipe bombs followed by multi-gun shootings - similar in style to the San Bernardino attack.

Marquez was tasked with purchasing weapons for both because of his Caucasian appearance - buying the first for $741.52 in November 2011 and the second for $758.75 the following February.

He also made multiple trips to gun ranges in Riverside and Los Angeles with Farook between February and June 2012 - but by the end of that year, Marquez had become disenchanted and stopped plotting terror.

That did not prevent him from marrying Farook's Russian sister-in-law Mariya Chernykh in November 2014 - a sham that brought him $200 a month.

Marquez, who was originally indicted on an immigration fraud charge relating to the marriage, has now had that count thrown out as part of his plea deal.

Chernykh, who has two children with US citizen Oscar Romero, pleaded guilty on one count of immigration fraud last month and will be sentenced this summer alongside her sister Tatiana Gigliotti and Farook's older brother Syed Raheel.

Marquez will be formally sentenced in August pending a pre-sentencing report and, according to court sources, is 'likely to be handed the maximum term' - 25 years in a federal prison.

Reacting to Marquez' plea, San Bernardino County Sheriff John McMahon said:

'December 2 will forever haunt the memories of the victims' families and the survivors who have lived through the tragedy.

'I pray today's guilty plea brings all of us a bit of justice.'