A week after a deadly bomb tore through central Bangkok, Thailand’s police chief says the frantic search for those responsible has been hampered by faulty security cameras.

The trail is growing increasingly cold seven days after the bombing, which killed 20 people – mostly Asian tourists – in the capital’s commercial heart and wounded scores more.

The main suspect is an unidentified man in a yellow T-shirt seen on CCTV placing a rucksack under a bench at the Erawan shrine minutes before the blast.

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He is still at large, with police publicly no nearer to naming him or his motivation.

National police chief Somyot Poompanmoung said his officers were working at “full capacity with no rest”.

But he added that the investigation was bedevilled by shortcomings, such as the fact that the majority of the city’s security cameras were not working.

“Sometimes there might be 20 CCTV cameras on a road but only five of them work,” he told reporters. “Another 15 might be broken for whatever reason.”

Here is the man Thai authorities are chasing in relation to the bombing.

Police also say they lack modern facial recognition technology to decipher who the bomber is from the grainy security camera footage.

Somyot is under intense pressure from both the public and the ruling junta to catch the perpetrators of an attack that sent shockwaves through the vital tourist sector.

On Monday bomb disposal officers rushed to a quiet street off one of the city’s main thoroughfares after a Myanmar migrant worker found a grenade on a construction site.

Police said the device had no link to their investigation and had been buried for some time.

Police are convinced the main suspect, who has been named in an arrest warrant as foreign, must have had help from a network, likely involving some Thais.

Last week junta chief Prayut Chan-O-Cha publicly bristled at any suggestions Thailand might ask for foreign investigators, though he has said he is open to technical advice from overseas.