Authorities on Saturday raided the home of Enrique Marquez, 29, who lived next door to Farook when he lived in Riverside, California.

Public records show that Enrique Marquez lived in the home next door to an old address of shooter Syed Farook.

Farook lived at the home with his wife, parents and siblings for several years before moving 17 miles to Redlands earlier this year.

While Farook has been described as 'shy' and an 'introvert' by his older sister Saira Khan, he had developed a longstanding friendship with Marquez.

The men are said to have shared a love of working on cars, neighbors told media.

Police believe that Marquez purchased the two modified 'assault-style' weapons that were used in Wednesday's massacre that left 14 dead.

Marquez, who has not been charged with a crime, admitted himself to a mental health facility in Long Beach, California, after Wednesday's attack and has been unavailable for questioning.

A neighbor told media that Marquez and Farook had been friends since childhood, but that they had grown distant in recent months. 

Marquez's stepfather, Gustavo Ramirez said that he and his wife hadn't heard from Marquez since Wednesday afternoon.

He said that it was unlike Marquez not to return home, and they were concerned.
 

Neighbor Sandie Emperador, 30, said she saw Marquez and Farook working together on many occasions fixing cars in Farook's garage.

Freddy Escamilla, 21, who lived across the street for most of his life, said the men 'had a common interest in mechanics and cars'.

'They would walk over to each other's yards and talk to each other, Escamilla told the Washington Post. 'It never seemed out of the ordinary.'  

Another neighbor, who declined to give her name, said she regularly saw Marquez visit Farook in his home and often saw the two of them laughing together, but that the friendship seemed to end abruptly about three years ago.

Around that time, she said, she was surprised to see them both standing on the street one day without acknowledging one another. After that, she said, she did not see them together again.

Several neighbors said they noticed about two years ago that Farook had begun wearing traditional Pakistani clothing - a long tunic. He also began growing his beard out.

FBI Director James Comey said on Friday that the gun buyer was 'not a suspect, at least at this point'.

Meredith Davis, with the Bureau of Alcohol, Tobacco, Firearms and Explosives, said that while the weapons were purchased legally, they had undergone a number of illegal adjustments afterwards.

One of the weapons had been modified to allow it to fire on fully-automatic mode, while the other was fitted with a large magazine and had the 'bullet button' removed to allow for quicker reloading.

California law bans the sale of weapons with magazines holding more than 10 rounds of ammunition, and while authorities say one of the rifles the couple used violated this law, it is not known exactly how many bullets it could hold.

The adjustments also mean the rifles, a .223 caliber Smith & Wesson M&P 15 and a .223 caliber DPMS A-15, would violate California's ban on assault weapons, Davis told the Wall Street Journal.

All rifles sold in the state must also be fitted with a 'bullet button' - meaning the magazine must be removed by using the tip of a bullet to trip the release mechanism, rather than a quick-release catch.

In total, Farook and Malik were carrying four guns - the two rifles and two semi-automatic handguns - and were in possession of 12 pipe bombs and around 4,500 rounds of ammunition.

Since the police raid, a makeshift sign that said to keep off of Marquez's property has since been put on his front lawn,Daily mail reported.

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