(CNN) Some Afghans fleeing their home country have arrived in the United States without paperwork amid the rapid pace of evacuations, according to two sources familiar with the situation, prompting additional screening and causing long delays.
There were Afghan nationals stuck at Washington Dulles International Airport -- where evacuation flights have landed -- over the weekend between eight and 12 hours, because of vetting challenges and lack of paperwork, while Customs and Border Protection officers sorted through how to handle the arrivals, one source said.
The approach from the administration has been "get as many people on the plane as you can, and we'll sort out the (immigration visa) stuff later," the source added, pointing to the rush to get people out of Afghanistan after the US-backed government there collapsed.
"Some people have landed with no documents whatsoever, creating a very challenging work environment for the officers," the source added.
Every person who is put on an evacuation flight to the US is being screened both in overseas staging areas -- outside of Afghanistan, like Qatar or Germany -- and upon arrival to the US, according to a Department of Homeland Security spokesperson.
DHS, which oversees the customs agency, did not respond to requests for comment on the number of people arriving without documents or whether security concerns had been raised about an Afghan after they landed in the US.
Even with missing paperwork, Customs and Border Protection has ways to identify people, but given the rush of evacuations, the source familiar said, "it's just a math game. The sheer numbers of people" make the possibility of slipping through the cracks "higher and higher."
Special Immigrant Visa applicants and "other vulnerable Afghans" are being flown to countries in Europe and Asia that have agreed to serve as transit hubs before flying to the US.
The Pentagon said Wednesday that of the approximately 7,000 evacuees from Afghanistan who have been processed in Europe before they are transported to the US, 52 have been flagged for further security screening but all have subsequently been cleared.
"Upon subsequent, more in-depth screening, all of those have been cleared," Gen. Tod Wolters, the commander of US European Command, said to reporters at the Pentagon. "We feel that we have a very good process in place that is DOD-centric, as well as State Department-centric with DHS."
The Justice Department's inspector general has cited the terror watch lists used for screening, which are maintained by the FBI, for a host of issues, including the late Sen. Ted Kennedy and Rep. John Lewis reporting problems boarding flights because their names were similar to those of people on the lists.
"The detection of security threats among Afghan evacuees is evidence that our security measures and robust vetting procedures are working," Ali Noorani, president and CEO of the National Immigration Forum, said in a statement Wednesday, urging the US to make getting its allies out of harm's way the number one priority.
DHS has deployed about 230 personnel from Customs and Border Protection, Immigration and Customs Enforcement, the Transportation Security Administration and the Coast Guard to Bahrain, Germany, Kuwait, Italy, Qatar, Spain and the United Arab Emirates to assist with processing, screening, vetting and helping to put together manifests for flights that are then transported to Customs and Border Protection as part of the process to vet individuals who land in the US, according to the spokesperson.
By the end of the week DHS expects that 350 personnel will be deployed across Europe and the Middle East for this effort.
Before boarding planes to the US, individuals undergo robust security checks that involve biometric and biographic screening conducted by US intelligence, law enforcement and counterterrorism professionals, a senior administration official said Tuesday.
There are three categories of evacuees: American citizens and lawful permanent residents (green card holders), Special Immigrant Visa applicants and their families, and other vulnerable Afghans who have been identified, according to the official.
After arriving in the US, evacuees are tested for Covid-19 at the airport. American citizens and green card holders can continue to their destinations, while others head to military bases around the US, the official said.
Several current and former US officials have pointed to the advances the government has made in vetting travelers in the 20 years since 9/11.
Screening is "now better than ever," the source familiar said, given technological advances, biometric capabilities, analytic capabilities and the ability to bring the entire US government and classified and unclassified data to bear.
A former US counterterrorism official compared the vetting effort to previous emergency screenings of refugees from Syria and Iraq.
Intelligence and law enforcement officials are always fearful of missing something in their vetting and that a terrorist could slip through.
"The challenging aspect is you can't predict the future. You can't tell when someone can go bad," the former official said.
In rare cases, refugees allowed to resettle in the US have been later discovered to have ties to terror groups and charged with lying to immigration authorities.
The difference now, the former official said, is that the US has better access to data from its 20 years in Afghanistan and cooperation with the former Afghan government. That wasn't the case with Syrian refugees fleeing the civil war there.