(CNN) -- A close relative of a missing Southern California family fears they may have been kidnapped.
"My gut feeling is telling me that they are being held against their will in some way," said Michael McStay, brother of Joseph McStay. "That's my personal opinion. That's what I think."
Joseph McStay, 40, his wife, Summer, 43, and their two children, Gianna, 4, and Joseph, 3, have not been seen or heard from since February 4. Concerned relatives contacted authorities, who then conducted a welfare check of the family's home on February 15, the San Diego County sheriff's office said.
An SUV belonging to the family was found abandoned near the Mexican border. The McStays' home is in Fallbrook, an unincorporated area about 45 miles north of San Diego.
"At this point it just seems to be a mystery," sheriff's Lt. Dennis Brugos said. "There is a chance the family could have voluntarily left and disappeared, but it's unlikely."
Michael McStay said he usually speaks to his older brother at least once a week.
"Every Friday he would call me, and we'd be talking for a while," Michael McStay said. "We were tight. We've always been tight."
The sheriff's office said evidence at the family home "suggested that the family had not left on a planned vacation," and that "the totality of the circumstances surrounding their disappearance was quite out of character for the family."
Brugos said the McStays' two dogs were left at the home unattended and without food or water.
"I think there were some food items left out that should have been refrigerated," Brugos added. "Those types of things sparked our attention, and on the chance that this turns into a criminal investigation, we want to be on the ground floor with this thing."
After the SUV was found in San Ysidro, two blocks from the Mexican border, "it was processed with a search warrant," Brugos said. "There was nothing in the car ... suggesting that this was some criminal act."
However, investigators are not ruling out the possibility of foul play and have assigned the case to the sheriff's homicide detail.
"There is nothing illegal about disappearing," Brugos said. "[But] in this particular case, according to relatives, it would be out of character just to disappear without notifying family members."
Brugos also said it was unlikely the McStays were targeted by a Mexican drug cartel. "There is nothing in Joseph or his wife's life or background that would suggest [or] that would put them in a high risk of any criminal activity," he said.
He said interviews with relatives and friends indicate no internal signs of marital trouble or conflict.
"Everyone has issues in every family, but as far as a family just disappearing, there is nothing to suggest they would do just that," he said.
Michael McStay said his brother is fun-loving and easygoing.
"He's creative, he's just, you know, people like to hang with him, both of them," Michael McStay said. "Summer is very intelligent, very educated, a heck of a mom. ... This doesn't add up."
Joseph McStay is self-employed, creating and installing indoor water features, Brugos said.
Brugos called the disappearance of the McStay family unsettling and unusual. "We've had cases where there have been incidents, but relatively quickly we are able to determine what happened," he said. "In this particular case, it's been quite a while."
Joseph McStay, 40, is 5 feet 9 and weighs 175 pounds. He has brown hair and brown eyes. Summer McStay, also known as Lisa Aranda-Martelli, is 43, 5 feet 5, 115 pounds, with brown hair and brown eyes.
Anyone with information on the whereabouts of the McStay family is urged to contact Sgt. Dave Martinez of the San Diego County sheriff's office at 858-565-5200. Anonymous calls can be made to 888-580-8477.