HOUSTON -- Another man transporting valuable jewels has been robbed, police told KPRC Local 2 Wednesday.

Houston police said the man was robbed as he got gas at the convenience store at the corner at Orem and Telephone roads in southeast Houston on Tuesday afternoon.

"(The attackers) jumped out of the car, two with knives, one with a gun," Sgt. Scott Atwood said. "(They) took him down to the ground (and) stabbed him in his right arm while taking the keys to his car. (They) went back to his car and took the briefcase out of his car."

Investigators said the man had about 600 karats worth of diamonds in the car. The jewels were stolen and the victim ran inside the store asking for help.

"Even if you see something bleeding, you also feel something bad like that. So, I was close my eyes. I could not look at," clerk Abel Vaz said.

Investigators said they have seen at least 16 similar robberies in the Houston area since February.

Detectives said they recently followed three men casing 15 jewelry stores around Houston. The Colombian nationals were arrested during a traffic stop on immigration charges and robbery charges are expected to follow, investigators said. Police said they hope someone will recognize the three and turn in whomever is responsible for the latest case.

"We believe that the group, the three suspects that were arrested Thursday, were just a small portion of a larger, organized group that are hitting jewelry salesmen," Atwood said.

Vaz said investigators planned to return to the store to get its surveillance tape.

Investigators said the stabbing victim went to the hospital, got stitches and flew home to California.

Anyone with information about the jewelry robberies is asked to call Crime Stoppers at 713-222-TIPS.