SAN ANTONIO (AP) — U.S. military investigators are asking law enforcement nationwide to be on the lookout for Afghan military members who went AWOL while training in Texas, though none is believed to be a national security threat, officials said Friday.

Air Force spokesman Gary Emery said 17 Afghans disappeared from Lackland Air Force Base in San Antonio one-by-one over the last 18 months, but a federal law enforcement official says seven have been accounted for. The official spoke on the condition of anonymity because privacy rules prevent disclosure of details about individual cases.

The men were vetted by the military and aren't believed to be connected to one another or to any terrorist group, Emery said. All had been studying English as a precursor to training sponsored by the U.S. and Afghan militaries.

"I don't think that anybody's really concerned that this is any sort of a plot or that they're looking to do anybody any harm," he said.

Emery said the disappearances were reported to immigration and federal law enforcement when they occurred, but an alert notifying law enforcement nationwide was issued Wednesday. Emery did not know what prompted the alert by the Naval Criminal Investigation Service, and a phone message and e-mail to an NCIS spokesman were not returned Friday.

The FBI referred all calls to NCIS.

The last of the men went missing in January. Their visas, driver's licenses and other travel documents were immediately revoked, but the men were issued base access cards that could allow them to drive on military bases, Emery said.

There was no information indicating the Afghans posed a threat, but the bulletin helps provide better investigative leads to immigration authorities, Immigration and Customs Enforcement spokesman Brian Hale said.

"Previous indications are that such foreign military deserters typically do so solely for prospects of a better life," he said.

Still, Texas Republican Sen. John Cornyn sent a letter Friday to Air Force Secretary Michael Donley calling the reports of the AWOL Afghans "alarming."

"I request an immediate report on how such a serious violation of security was allowed to happen not once, but on 17 separate occasions, and why safeguards were not put in place to prevent repeated incidents," he said.