FBI agents on Thursday were searching the California townhome of the brother of San Bernardino terrorist Syed Rizwan Farook amid lingering questions about a possible third shooter.

The brother, Syed Raheel Farook, is a Navy veteran who earned medals for fighting global terrorism.

Agents were carrying out a search warrant as part of an ongoing investigation, FBI spokeswoman Laura Eimiller confirmed. No arrests had been made.

Raid: FBI and Homeland Security investigators carry a computer tower and manila envelopes from the Corona, California, home of Syed Raheel Farook, the brother of one of the San Bernardino shooters

Navy man: Syed Raheel, pictured here with his wife, Tatiana Chernykh, is a decorated US Navy veteran

Reporters and onloookers stand across the street from the Farook home on Forum Way in Corona Thursday morning

A local police cruiser drives by the townhouse of Raheel Farook, which is also home to his father

Eimiller declined, however, to comment on where the search was happening or whether it had anything to do with the deadly shooting. An affidavit in support of the warrant was under seal.

Syed Rizwan Farook's brother and sister-in-law, Tatiana Chernykh, live in a home in the 1700 block of Forum Way in Corona.

Farook and his wife, Tashfeen Malik, killed 14 people and wounded more than 20 others in the December 2 terror attack at a meeting of his coworkers in San Bernardino, about an hour east of Los Angeles.

Syed Raheel Farook was in the Navy from 2003 to 2007, military records show. During his stint, he received the Global War on Terrorism Expeditionary Medal and the Global War on Terrorism Service Medal, among other awards.

After going through training in the family's native Illinois, Syed Raheel Farook served for three years aboard the aircraft carrier USS Enterprise as an information system technician.

Farook and Chernykh's neighbor told the station KTLA the FBI descended on the couple's home at around 6.30am on Thursday and left more than four hours later carrying stacks of manila envelopes, bags and a desktop computer.

Terror duo: Syed Raheel's brother, Syed Rizwan (right) and his sister-in-law Tashfeen Malik (left) carried out a mass shooting on December 2 in San Bernardino that left 14 dead and more than 20 wounded

In this December 19, 2015, photo, Syed Raheel Farook, left, brother-in-law Farhan Khan, second from left, and mother, Rafia Farook, third from left, leave the Redlands home of Farook and Malik

The couple live at the Forum Way address with their child, Mr Farook's father, and until recently, his mother, Rafia Farook, who had moved out of her slain son and daughter-in-law's Redlands home after the terrorist attack.

Neighbor Stacy Mozer said that Syed Raheel and his wife are ideal neighbors and very pleasant people.

Mozer revealed that the family's home was searched twice after the December terror attack and that authorities previously broke down the front door. Shards of wood and damage to the lock were still visible Thursday.

Tatiana Chernykh's sister, Marya, was married to Syed Rizwan Farook's former neighbor and friend Enrique Marquez,. who has been charged with plotting failed attacks with Farook in 2011 and 2012, and buying two AR-style rifles used in the San Bernardino massacre.

He is also facing an immigration fraud charge stemming from his sham marriage to the Chernykh sister.

The search warrant was executed a day after Apple CEO Tim Cook said the company would fight federal government efforts to help the FBI hack into an iPhone that had been used by shooter Syed Farook.

A federal magistrate ordered Apple to help the FBI get into the phone but Cook said doing so would mean building a 'backdoor' that would bypass digital locks protecting consumer information on iPhones. He says the software would be too dangerous to create.

Meanwhile, it was revealed this week, more than two months after the ISIS-inspired mass shooting in California, that the FBI still has not ruled out the possibility that Farook and Malik had a third accomplice.

Lingering questions: More than two months after the deadly attack, which ended with Farook and Malik being shot dead by police in their rented SUV (pictured), officials still have not ruled out the possibility that the couple had an accomplice

Victims: A composite photo of all 14 victims from 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

Law enforcement officials told ABC News that in the immediate aftermath of the shooting, some people reported seeing three gunmen at the Inland Regional Center, not two.

'What we have confirmed is evidence indicating that two weapons were fired at the Inland Regional Center,' Laura Eimiller, the FBI Los Angeles spokeswoman, told ABC. 'But in the absence of video it’s something you can’t entirely rule out until every question is answered. There’s still unanswered questions.'

Eyewitness Sally Abdelmageed told the press back in December that she saw a trio of shooters in black military-style clothing and tactical vests.

However, Lt Mike Madden, the first San Bernardino police officer to respond to the scene of the shooting, said there was no evidence to support the theory that there were three suspects.