Napolitano spoke at a news conference Tuesday at which she released detention reform recommendations based onby her former detention adviser, who resigned last month to become commissioner of New York City's jails.
The recommendations were preceded by years of rapid growth in immigration detention - from 7,500 beds in 1995 to more than 30,000 this year - along with reported abuses such as medical neglect of detainees and denial of due process.
"Our immigration enforcement measures continue unabated," Napolitano said.
She said Immigration and Customs Enforcement, the part of the Homeland Security Department overseeing detention, would submit to Congress by this fall a plan for detention alternatives, including electronic ankle bracelets.
Sen. Chuck Schumer, chairman of the Senate Judiciary Committee's immigration subcommittee, urged ICE to use ankle bracelets rather than creating new facilities.
Napolitano and John Morton, the assistant secretary for ICE, say they plan to confine immigrants awaiting deportation according to the risk they pose. Morton will also consider using former hotels, nursing homes and other sites to hold immigrants who do not pose a safety or flight risk.
The Associated Press obtained copies of the planned recommendations Monday. Officials gave a preview of the reforms in August.
Not all detainees are illegal immigrants. Some entered the country legally and committed a crime making them deportable. Others are seeking asylum. In some cases, citizens are detained. Many people in detention have not been convicted of a crime.
ICE does not have authority over U.S. citizens, even though some U.S. citizens are held for possible deportation. ICE declined to say how the reforms might affect detainees who claim U.S. citizenship.
"Meaningful reform of the system must focus not only on the conditions under which immigrants are being detained, but on why they are being detained in the first place, often for prolonged periods of time," said Judy Rabinovitz, deputy director of the American Civil Liberties Union's Immigration Rights Project.
Immigration officials believe some alternatives could cut the cost of detention, which reached nearly $2 billion in 2008.
The agency says possible alternatives will cost only about $14 a day compared with about $100 a day to house immigrants in a detention center. However, alternatives are not always automatically cheaper, the officials said.
ICE also plans to put 50 federal employees in detention centers where more than 80 percent of immigrants are held to monitor and improve conditions. It was unclear how much access the public would have to the monitors' reports on detention conditions.