The San Bernardino attack “shows that a small number of people determined to plan but not boast can get away with it,” said Patrick Skinner, a former case officer with the Central Intelligence Agency. “In this way, terrorism is exactly like any other crime.”

The couple, who were killed Wednesday in a gunbattle with police, apparently sought to hide evidence that might connect them to others, law-enforcement officials said. Two relatively new cellular phones were found smashed in a garbage can and a computer in their townhouse was missing a hard drive. Investigators have subpoenaed email service providers to retrieve any communications.

Some questioned whether U.S. and local law-enforcement officials may have missed signs that the couple had become radicalized. Mr. Farook had communicated with at least one FBI terrorism suspect, for instance. But U.S. law enforcement agencies had no case files on either Mr. Farook, an environmental-health specialist who worked for San Bernardino County, or his wife, whom Mr. Farook married during a trip to Saudi Arabia, where she had lived most of her life.