AHMADABAD, India — Police defused 18 bombs found Tuesday near the main diamond markets in the city of Surat and issued a sketch of a young man believed to be linked to one of two explosives-filled cars discovered there.

The announcement came as authorities in a Mumbai suburb probed ties to a series of blasts over the weekend that killed 42 people and wounded 183 in Ahmadabad, about 175 miles north of Surat. An obscure Islamic militant group claimed responsibility for the Ahmadabad attack.

"I request you not to go to crowded places unnecessarily," Surat Police Commissioner R.M.S. Brar told the public during a news conference, according to the Press Trust of India news agency.

Authorities said that four cars — two used in Saturday's Ahmadabad attack and the two found in Surat on Tuesday — were stolen in July from a Mumbai suburb, Navi Mumbai.

"Once we find the people who stole the cars, it will give us further clues about the blasts," Navi Mumbai police chief Ramrao Wagh told The Associated Press.

Police said they believe the bombers used Navi Mumbai as the headquarters to plan the attack because they believed their activities would likely go undetected in the nondescript suburb.

India has been hit repeatedly by bombings in recent years. Nearly all have been blamed on Islamic militants who allegedly want to provoke violence between India's Hindu majority and Muslim minority, although officials rarely offer hard evidence implicating specific groups.

Meanwhile, in the neighboring state of Rajasthan, police defused three crude bombs hidden inside plastic containers in a village about 190 miles north of Jaipur, said Harilal Sharma, a local senior police official.

The latest developments come just days after 22 bombs tore through the historic city of Ahmadabad in Gujarat state in western India on Saturday. The death toll has been lowered to 42 from 45 because several cases were reported twice amid the confusion, said H.P. Singh, a senior Ahmadabad police officer.

"In the name of Allah the Indian Mujahideen strike again! Do whatever you can, within 5 minutes from now, feel the terror of Death!" said an e-mail from the Islamic group sent to several Indian television stations minutes before the blasts began.

The e-mail's subject line said "Await 5 minutes for the revenge of Gujarat," an apparent reference to 2002 riots in the western state that left 1,000 people, mostly Muslims, dead. The historic city of Ahmadabad was the scene of much of the 2002 violence.

Ahmedabad and Surat are about 175 miles apart in Gujarat state; Mumbai is in another state bordering to the south.

Authorities on Tuesday also were investigating the computer of a 48-year-old American citizen living in Mumbai to find out if an e-mail claiming responsibility for the attack was sent from it, or if unknown attackers accessed his wireless Internet connection.

Police seized Kenneth Haywood's computer Monday after tracing an e-mail claiming responsibility for the attack to the machine. Police said Tuesday that Haywood was not a suspect and it appeared the bombers had accessed his wireless network connection to send the e-mail.

"He has never been detained, but we have called on him and questioned him as part of the investigation," said Parambir Singh, a senior officer in the Anti-Terrorism Squad. "He has said his e-mail ID was hacked and evidence we have gathered shows that his network was used to forward the mail."

Singh said anyone on the two floors below Haywood's 15th floor apartment could have accessed his network.

The Hindustan Times newspaper quoted Haywood, a manager at the consultancy company Campbell White, as saying that the telephone technician who set up his Internet connection had insisted he not change his default password.

Associated Press Writer Matthew Rosenberg in Ahmadabad and Ramola Talwar Badam in Mumbai contributed to this report.