(CNN) -- The Department of Homeland Security has ordered U.S. Customs and Border Protection agents to verify, "effective immediately," that every foreign student who wants to enter the United States has a valid student visa, a U.S. government official told CNN on Friday.
The memo went out earlier this week as part of an effort to reform the student visa system, said the official, who spoke on condition of anonymity. The effort is meant to provide the agency with real-time updates on all relevant information, the official said.
News of the memo follows reports that two friends of Boston Marathon bombing suspect Dzhokhar Tsarnaev, who are now charged in connection with the attack, may have been in the country on student visas that were no longer valid.
Dias Kadyrbayev and Azamat Tazhayakov, both 19 and from Kazakhstan, were charged this week on suspicion of obstruction of justice in connection with the case.
They were already in federal custody on suspicion of violating the terms of their student visas when they were charged in the bombings case.
The pair started at the University of Massachusetts Dartmouth in 2011, along with Tsarnaev.
Kadyrbayev was not enrolled at the time of his arrest last month. His lawyer, Robert Stahl, told CNN his client had not been attending classes on a regular basis, as required by his visa.
Tazhayakov's student status is less clear. According to the university, Tazhayakov is enrolled but was suspended pending the outcome of the current case.
At an immigration court hearing this week, however, the court heard that Tazhayakov's status with the school was terminated January 3, according to a U.S. government official.
The court also heard that Tazhayakov went to Kazakhstan in December 2012 and was allowed to return to the United States on January 20, despite the apparent lapse in his visa status, the same government official said.
That news has outraged U.S. Rep. Michael McCaul, head of the House Committee on Homeland Security.
"The fact that a foreign national was able to re-enter the U.S. with what appeared to be a valid student visa, while Customs and Border Protection officers were unaware that his visa status had become invalid, represents a serious hole in our national security," McCaul told CNN. "The front-line CBP officers did not have access to the system that would've informed them of a change in legal status."
McCaul said it indicates "disconnects" and problems with information-sharing among Homeland Security agencies.
Sen. Chuck Grassley, ranking member of the Senate Judiciary Committee, said Friday he also has concerns about gaps in the immigration system.
"It's surprising that the administration isn't already verifying that any student coming into the country has a valid student visa," Grassley said. "What's more concerning is that nearly 12 years after the 9/11 terrorist attacks, we haven't fixed the problems with identifying visa overstays."
Grassley sent a letter and list of questions Friday to Homeland Security Secretary Janet Napolitano, asking her about the apparent visa lapse with Tazhayakov and other possible loopholes.
CNN's Jake Tapper, Joe Johns and Ted Barrett contributed to this report.