By Mary Beth Sheridan WASHINGTON — The federal government is waging part of the war against terrorism with a seemingly innocuous weapon: immigration law. In the past two years, officials have filed immigration charges against more than 500 suspects who have come under scrutiny in national security investigations, according to previously undisclosed government figures. Whereas terrorism charges can be difficult to prosecute, Department of Homeland Security officials say immigration laws can provide a quick, easy way to detain people who could be planning attacks. Authorities have used routine charges such as overstaying a visa to deport suspected supporters of terrorist groups. ”It’s an incredibly important piece of the terrorism response,” said Michael J. Garcia, who heads Homeland Security’s Immigration and Customs Enforcement, or ICE. And although immigration violations might seem humdrum, he said, ”They’re legitimate charges.” Muslim and civil liberties activists disagree. They argue that authorities are enforcing minor violations by Muslims and Arabs, while ignoring millions of other immigrants who flout the same laws. They note that many of those charged are not shown to be involved in terrorism. ”The approach is basically to target the Muslim and Arab community with a kind of zerotolerance immigration policy. No other community in the United States is treated to zero-tolerance enforcement,” said David Cole, a Georgetown University law professor and critic of the government’s antiterrorism policies. Before the Sept. 11, 2001, attacks, immigration agents were minor players in the world of counterterrorism. That changed during the investigation of the hijackings, when 768 suspects were secretly processed on immigration charges. Most were deported after being cleared of connections to terrorism. Unlike that controversial roundup, most of the recent arrests have not involved secret proceedings. Still, they can be hard to track. A few cases have turned into high-profile criminal trials, but others have centered on little-known individuals processed in obscure immigration courts, with no mention of a terrorism investigation. In some cases, the government ultimately concludes a suspect, while guilty of an immigration violation, has no terrorism ties. Authorities are often reluctant to disclose why an immigrant’s name emerged in a national security investigation, because the information is classified or part of a continuing inquiry. Homeland Security officials turned down a request for the names of all those charged in the past two years, making it difficult to assess how effective their strategy has been at thwarting terrorism.