OCEANSIDE, Calif. -- Authorities are investigating a string of possibly linked robberies at Oceanside gas stations and liquor and convenience stores in recent weeks.

Six of the eight crimes involved an armed white man wearing a white face-covering, according to the Oceanside Police Department.

The latest three robberies occurred Sunday, over a span of 19 hours, police said.

Those instances involved two masked gunmen, one of whom wore a gray hooded sweatshirt and either a bandana or a torn T-shirt over his face.

His stocky companion is black and about 5 feet 11 inches tall, also with cloth material covering his face, authorities said.

The first in the trio of Sunday robberies took place at 2:52 a.m. at a 7-Eleven on Oceanside Boulevard; the second about an hour later at an AM/PM in the 3800 block of Plaza Drive; and the third about 9:45 p.m. at a market on Morse Street, Sgt. Leonard Mata said.

About 12:20 a.m. Saturday, a man wearing a blue hooded sweatshirt held up a North Redondo Drive liquor store and was last seen getting into a white Mazda sedan driven by another male, he said.

Two robberies also took place in Oceanside on Friday, approximately 30 minutes apart.

The first was at 10:08 p.m. at a gas station in the 1700 block of South Coast Highway. At 10:41 p.m., a liquor store on the same street several blocks away was held up, Mata said.

In both instances, a white gunman with a white bandana, a blue hooded sweatshirt and blue jeans demanded cash from the clerks, Mata said, adding a robber with a similar description hit the same South Coast Highway liquor mart on June 8.

A June 3 robbery at an unspecified location may be related to the series, police said.

No arrests have been made.