Agent William Elwell of the FBI field office in Albuquerque said no arrests had yet been made but investigations were ongoing after the seizure of material from a home at a trailer park.

"We are still analysing the material and at the moment all I can say is that we believe there are grounds for the investigation to continue," Elwell told AFP by telephone. "I can't comment on what we have discovered so far."

The possible security breach at the nuclear weapons research centre came to light after Los Alamos police were called to a domestic disturbance on October 17 and found evidence of drug use.

That led to execution of a search warrant which discovered materials believed to have come from the Los Alamos lab, where the atomic bomb was first developed by scientists during WWII.

"During the course of the search, officers realized that some of the items seized appeared to belong to the Los Alamos National Laboratory," the Los Alamos County Police Department said in a statement.

Local media reported the suspicious materials belonged to a data clerk at the lab, and said police unearthed three computer memory sticks in the raid.

A separate FBI search warrant was carried out on October 20 but Elwell would not disclose what material federal investigators were looking at.

Los Alamos officials quote by local media stressed that the search related to a former employee whose contract expired several weeks ago.

A spokesman for the laboratory appeared to play down the significance of the employee's job in comments to The New Mexican newspaper.

"It's clear this person was not a scientist," spokesman Kevin Roark said, declining to elaborate on the data or materials that may have been found.

"I can't speak to the nature of the information or the materials. It's a matter of security."

Los Alamos National Laboratory Director Michael Anastasio said in a statement Wednesday "we will take immediate and decisive action to address any safety and security issues. This matter is a reminder that constant vigilance by everyone is necessary to ensure a safe and secure Laboratory."

It is not the first time that Los Alamos has been at the centre of a security alert.

The most notorious recent case involved Taiwanese-American scientist Wen Ho-Lee, who was accused of stealing nuclear secrets for China.

Investigators later dropped the original charges but Lee pleaded guilty to improperly handling classified data as part of a plea deal.

Two years ago another security alert was sparked when it emerged that two computer disks containing sensitive information had gone missing.

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