Investigators probing the Ahmedabad [Images] serial bomb blasts, which claimed 55 lives, have prepared sketches of the three suspects as Gujarat remains on high-alert, a senior police official said on Saturday.

"We have prepared three sketches of suspects based on the description given by an eye-witness who saw them planting the bombs in different parts of the city," Joint Commissioner of Police Ashish Bhatia said.

"We have decided to not make those sketches public at this stage as it will hamper the probe," he added.

The police are conducting regular combing operations and security has been tightened in vital locations of the state.

On the forensic report of the bombs, Bhatia said, "It seems that the material used to make the bombs came from outside the state. But it is likely that they were assembled here. Since items like batteries and watches are found locally, we believe that the assembly could have been done here."

Victims admitted at the Ahmedabad Civil Hospital have described the series of events leading to bomb blasts to the police officials, said Bhatia.

"We have seen the television footage and recorded statements of eye-witnesses who were present near the trauma centre of the hospital," Bhatia said.

Meanwhile, security at railway stations, bus stations, temples and other public places remains tight.

Railway authorities have stopped issuing platform tickets at Ahmedabad, Vadodara and Surat [Images] stations as a precautionary measure. Vehicles entering and leaving the city are being checked regularly.

The police have also tightened the noose on workers of the Students Islamic Movement of India.

Several former activists of the banned organisation in Ahmedabad, Bharuch, Surat and Vadodara have been questioned and their movements put under watch, police said.

The opposition Congress party has demanded a probe by the Central Bureau of Investigation into the terror attacks.