Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, on Monday pressed the Department of Homeland Security and the State Department for more information on the alleged ISIS killer arrested in northern California last week, asking how he could have been granted refugee status.

The FBI Joint Terrorism Task Force arrested Omar Ameen, 45, following an Iraqi warrant for his arrest and extradition request, officials said in a news release from the Justice Department. Ameen's refugee status was approved under an Obama-era resettlement program in June 2014.

But before arriving in the U.S. in November 2014, prosecutors say, Ameen went back to Iraq and killed a police officer while fighting for ISIS.

“We need to better understand how someone with ties to terrorism and wanted for murder could possibly be granted refugee status and allowed to enter American communities," Grassley wrote in a letter to Secretary of State Mike Pompeo and DHS Secretary Kirstjen Nielsen. "This disturbing case underscores the importance of thoroughly vetting refugees before granting admission to the United States."

SUSPECTED ISIS MEMBER CAPTURED IN SACRAMENTO

Grassley's letter set a Sept. 3 deadline for DHS officials to provide written information on Ameen's immigration history, relevant Visa Security Unit vetting documentation and all records about his travel into and out of the U.S.

The letter also requested that the State Department turn over information as to whether Ameen applied for a visa, whether he ever was denied a visa and why, as well as any additional administrative processing that his application received.

Ameen allegedly arrived in Rawah, Iraq, “with a caravan of ISIS vehicles," after the terror network gained control of the town on June 21, 2014, as the group made its way to the Rawah police officer's home, officials said. The following day, multiple ISIS members, including Ameen, allegedly shot the officer.

"This disturbing case underscores the importance of thoroughly vetting refugees." — Sen. Chuck Grassley, R-Iowa

“Ameen then allegedly fired his weapon at the victim while the victim was on the ground, killing him,” officials said.

The suspect is accused of being a member of ISIS and Al Qaeda in Iraq (AQI), as well as carrying out tasks for the groups, “including helping to plant improvised explosive devices,” according to the news release.

Following the alleged incident, Ameen relocated to Sacramento from Iraq “as a purported refugee” and did not mention his alleged ties to the terrorist organizations when seeking refugee status and a green card in the U.S., officials said.

Ameen also attempted to apply for refugee status in Turkey in 2012, according to Grassley, but "returned to Iraq to engage in terrorist activities while his application was pending."

"It’s difficult to judge what mistakes may have been made and what policy changes might be necessary to prevent it from happening again without more information,” Grassley added. “I’m grateful for the work of agents and officials at the FBI, the Justice Department and elsewhere who appropriately removed this potential threat, but I am concerned that an active member of two foreign terrorist organizations was able to conceal his past and slip into the United States undetected.”

Ameen could face the death penalty after he is extradited to Baghdad under a treaty the U.S. has with Iraq, court documents filed in the U.S. by Iraqi prosecutors say.

Fox News' Elizabeth Zwirz contributed to this report.