Some once-furloughed federal workers have returned to their jobs this week, responding to emergencies and changing policies as the partial government shutdown dragged on for a second week.

At least four agencies and one lawmaker have recalled personnel, citing various legal and philosophical justifications.

On Tuesday, the CIA announced that it would call back thousands of employees because of concerns that an extended shutdown would start to threaten national security.

By law, certain functions must continue during a partial government shutdown and certain agencies are allowed to operate with employees who must work while their pay is delayed. Those functions include national security issues and generally the safety of both life and property. Benefit payments such as Social Security also must continue.

Managers determine how to execute those functions and the staffing required.

“Activities that could be suspended for a brief period of time without threatening the safety of life or property will need to be restarted again,” said an official from the White House Office of Management and Budget.

CIA spokesman Dean Boyd declined to reveal how many employees the agency has furloughed, citing the classified nature of its work.

On Tuesday, the Centers for Disease Control and Prevention said it had brought back nearly a dozen foodborne-illness experts to help with a salmonella outbreak that had sickened about 280 people in 18 states. The recalled employees include epidemiologists and analysts who can address the problem, apparently caused by raw poultry distribution in California, Oregon and Washington state.

About 9,000 of the CDC’s 13,000 workers remained on furlough, spokeswoman Barbara Reynolds said Tuesday.

The Federal Emergency Management Agency called back more than 100 employees last Friday to prepare for a possible hurricane heading for the Gulf Coast. The storm dissipated Sunday night before reaching shore.

FEMA has since “re-furloughed” most of those workers, agency spokesman Daniel Watson said.

Certain agencies may recall employees in response to extreme circumstances, but that hasn’t stopped the shutdown from taking its toll, said the OMB official, who spoke on the condition of anonymity.

“It goes without saying that a government shutdown, particularly one of this length, puts tremendous strain on agencies, given that their programs are not designed to operate in the absence of appropriations,” the official said.

Amid the largest recall of federal employees, the Pentagon announced Saturday that it was ordering nearly all of its 350,000 furloughed civilian personnel back to work. Defense Secretary Chuck Hagel said the employees provide direct support for the military, qualifying them to return under the agency’s interpretation of the military-pay bill that President Obama signed last week. That law authorized continuing to pay “excepted” Defense employees.

At least one member of Congress also has brought back workers. Rep. Mark Sanford (R-S.C.) said Monday that he recalled all of his furloughed staff members “in the interest of the taxpayer” after the House passed legislation to provide back pay for sidelined employees. “I think common sense would dictate that now that it’s clear everyone will be paid, that they should work — anything less would amount to an extended taxpayer subsidized vacation,” he said in a statement. The Senate has yet to pass the bill, which Obama has pledged to sign if it reaches his desk.

Some agencies are moving in the opposite direction, anticipating that they will have to take more workers off the job over time. The Department of Veterans Affairs has furloughed about 63 percent of its benefits-administration employees, but it warned that the figure could increase to 95 percent by the end of the month as the division runs out of money for benefit checks.

The State Department has sent home less than 1 percent of its roughly 70,400-member workforce, but agency spokeswoman Brenda Greenberg said that “thousands more could be furloughed as carryover funds are exhausted.”

The Office of Personnel Management’s shutdown rules do not say how much notice an agency must give employees who are required to return. Guidelines for pay remain the same as they are for all non-furloughed personnel who do not work for the Defense Department, meaning they will be compensated once funding is available.