Thailand's police chief said Monday the investigation into last week's bomb blast has been hampered by broken security cameras in central Bangkok along the main suspect's getaway route.

National police chief Somyot Poompanmoung said that police were trying to "put pieces of the puzzle together" but had to use their imagination to fill holes where street side security cameras were broken and unable to record his movements.

One week after last Monday's bombing at the capital's revered Erawan Shrine, which left 20 people dead and scores injured, police appeared no closer to tracking down suspects or determining a motive for the attack.

Police have released an artist sketch of the prime suspect who is seen in security camera footage from the open-air shrine leaving a backpack at a bench and walking away. The explosion takes place 15 minutes later.

But the images are blurry and after the suspect leaves the scene, the security cameras were broken at key spots along his suspected path, Somyot said.

"Sometimes there are 20 cameras on the street but only five work," Somyot said, openly frustrated as he spoke to reporters. "We have to waste time putting the dots together."

"The footage jumps around from one camera to another, and for the missing parts police have to use their imagination," he said, adding that the Thai police lack the sophisticated equipment seen in popular TV crime shows, like CSI.

"Have you seen CSI?" Somyot asked reporters. "We don't have that," he said, referring to high-tech equipment that can render blurry footage clear.

He said that Thailand has "asked for cooperation from countries with better equipment and technology."

On Friday, Prime Minister Prayuth Chan-ocha said that he had received offers of assistance from the U.S. Embassy in Bangkok and had assigned his deputy "to cooperate on borrowing equipment that includes facial-recognition technology.

However, Prayuth ruled out working with U.S. investigators, insisting Thais can do the job.

On Sunday, Somyot said that investigators will "need some luck" to catch the perpetrators whom are suspected to have already left the country.