The Pima County Sheriff’s Department and the FBI said on Wednesday they are continuing to follow up on all 18,000 calls to investigators related to the disappearance of Nancy Guthrie.

Up to 4,000 calls to investigators have been made since the FBI released doorbell-camera images and video of an "armed individual" apparently tampering with a security camera at the front door of the 84-year-old woman's house near Tucson, Arizona, authorities said.

"We thank the media and the public for the thousands of tips submitted through the various tip lines," the sheriff's office and the FBI said in a joint statement.

Several hundred detectives and agents are assigned to this case, and authorities said, "You will continue to see increased law enforcement activity throughout Pima County as the investigation expands."

-ABC News' Aaron Katersky

The individual detained for questioning in connection with the disappearance and abduction of Nancy Guthrie had been on the radar of the FBI and Pima County Sheriff’s Department before Tuesday's public release of the images of a masked subject at her front door, law enforcement sources told ABC News.

The individual, who identified himself as Carlos, was detained for questioning in Rio Rico, Arizona, south of Tucson, after the FBI identified him by using a series of electronic investigative tools, including cell phone usage information and traffic data, the sources said, without specifying the exact techniques.

A local judge from Santa Cruz County, Arizona, signed the warrant for the search of Carlos' house, which was carried out overnight.

Carlos was questioned for several hours before being released without charge. He is under no law enforcement restrictions.

Separately, investigators are studying every pixel of the new video the FBI released publicly on Tuesday to try to find any identifying feature, including the apparent weapon and characteristics of what the subject was wearing, according to the sources.

-ABC News' Aaron Katersky and Trevor Ault

The FBI agents are conducting an "extensive search" on Wednesday along multiple roads in the Catalina Foothills neighborhood surrounding Nancy Guthrie's Tucson-area home, the bureau's Phoenix office said.

“We are asking the media and motorists to follow all traffic laws and to remain especially cautious when passing law enforcement near roadways,” FBI Phoenix said in a statement.

The FBI said it continues to offer a $50,000 reward for information leading to the recovery of Nancy Guthrie.

A delivery driver who said he was detained by investigators in the Nancy Guthrie disappearance claimed in an interview that law enforcement officers had been following him.

The man, who only wanted to be identified as Carlos, said he was driving his Nissan Rogue on Tuesday evening in Rio Rico, Arizona, about 50 miles south of Tucson, when he noticed he was being trailed by a law enforcement vehicle.

"They were following me," Carlos told ABC Phoenix affiliate station KNXV. "And once I saw they were following me, I pulled over. They didn't even have to [make] a traffic stop. I got out of the car and they arrested me as soon as I got out of the car."

Carlos said he was not immediately told why he was being detained. But when an FBI agent and a Santa Cruz County deputy sheriff arrived at the scene, "They told me I was being detained for kidnapping."

"And I asked them, kidnapping of who?" said Carlos, adding that he was told of Nancy Guthrie.

He said that after being detained for several hours and having his vehicle thoroughly searched, he was released.

"Deputies detained a subject during a traffic stop south of Tucson," the Pima County Sheriff's Office said in a statement, confirming the man was "questioned in connection to the Nancy Guthrie investigation."

A woman who described herself as Carlos' mother-in-law told KNXV that investigators, armed with a search warrant, swarmed her home on Tuesday night, claiming they got a tip that Nancy Guthrie, the mother of "Today" show host Savannah Guthrie, was inside her house.

"I told them, 'You can go in and search my house,'" the woman said. "There's nobody there."