Federal investigators in Northern California routinely used a sophisticated surveillance system to scoop up data from cellphones and other wireless devices in an effort to track criminal suspects — but failed to detail the practice to judges authorizing the probes.

The practice was disclosed Wednesday in documents obtained under the Freedom of Information Act by the American Civil Liberties Union of Northern California.

The investigations used a device known as a StingRay, which simulates a cellphone tower and enables agents to collect the serial numbers of individual cellphones and then locate them. Although law enforcement officials can employ StingRays and similar devices to locate suspects, privacy groups and some judges have raised concerns that the technology is so invasive — in some cases effectively penetrating the walls of homes — that its use should require a warrant.

The issues, judges and activists say, are twofold: whether federal agents are informing courts when seeking permission to monitor suspects, and whether they are providing enough evidence to justify the use of a tool that sweeps up data not only from a suspect’s wireless device but also from those of bystanders in the vicinity.

In Northern California, judges expressed concerns about the invasive nature of the technology.

Upon learning that StingRay technology was being used in the field without specific permission, Miranda Kane, then chief of the Northern California U.S. attorney’s office’s criminal division, ordered that all surveillance applications and proposed orders be reviewed before being submitted to a magistrate judge.