What the agents are searching for was not immediately known.
The park is a few miles north of the Inland Regional Center.
Fourteen people were killed and 21 others were injured last Wednesday when Syed Farook and his wife Tashfeen Malik opened fire during a holiday banquet hosted by Farook's co-workers with the county health department.
Farook, 28, and Malik, 29, were killed hours later in a shootout with police, leaving behind a 6-month-old daughter.
The FBI has said that Malik declared her allegiance to the Islamic State group in a Facebook posting shortly before the attack.
FBI Director James Comey told the Senate Judiciary Committee on Wednesday that the two were radicalized well before Malik came to the U.S. on a fiance visa and they had discussed jihad and martyrdom as early as 2013.
Meantime, new revelations from officials and public records show a much deeper connection between Farook and the man who purchased the weapons used in the mass shooting, 24-year-old Enrique Marquez.
Marquez and Farook had plotted a terrorist attack together back in 2012, ABC News reports. But Farook shelved the attack after the FBI made a round of terror-related arrests in different cities, Eyewitness News has learned.
The two were friends for years and became family last year with a sister-in-law in common. They were listed as witnesses on the marriage license when Farook's brother, Raheel, wed a Russian woman in 2011.
MORE: San Bernardino attack leads to questions about another marriage
Three years later, Raheel Farook and his wife, Tatiana, were witnesses to Marquez's marriage to her sister, Mariya Chernykh, according to Riverside County records.
The FBI is currently trying to verify Marquez's mental stability after he checked himself into a mental facility two days after the attack.
The Associated Press contributed to this report.