KARACHI, Pakistan (CNN) -- Karachi police have detained four suspects in connection with a series of blasts that injured dozens of people in the city's Pashto neighborhoods, a senior police officer told CNN on Tuesday.
"We have got some clues (from) motorcycles and from the cars that were stolen and later on used in the blast(s)," Wasim Ahmed said.
"We have some leads from that, and we have rounded up some suspected persons from it."
Karachi -- Pakistan's largest and most populous city -- remains on high alert after at least five bombs exploded within 30 minutes of each other Monday in western Karachi.
About 40 people were wounded. Initial reports of one fatality turned out to be inaccurate, a police source said.
Most of those injured were treated and released, Sindh Home Minister Zulfiqar Mirza said Tuesday. Five or six of the injured were hospitalized, he said. Army rangers and police personnel remain stationed throughout the port city, according to Sindh Province police. Karachi is the capital of Sindh Province.
Provincial, local and federal investigators are trying to determine who is responsible for the bombings, which took place in neighborhoods populated mostly by Pakistanis from the northern and tribal areas of Pakistan.
At least three of the bombs were placed on vehicles -- a bicycle, a motorbike and a truck, police said.
"A white car was parked here, and I was told by residents that someone had come and parked this car here," said Abdullah Akbar, who witnessed the attack.
"As soon as I moved, there was a blast. Children who were playing were injured, and you can see blood spots on me as well.
"Soon after, someone on a motorbike threw a grenade or something, and there was another blast and more children were injured."
Police are investigating whether there was any ethnic motivation behind the attack. The bombs were made of low-intensity explosives, and police believe the motivation was civil unrest, not mass casualties.
The blasts happened 24 hours after a suicide attack in Islamabad. At least 13 police officers were killed in Sunday's attack, the deadliest in Pakistan's capital this year.
The suicide bomber detonated just blocks from a massive rally commemorating the first anniversary of a deadly raid on Islamabad's Red Mosque.
More than 100 people died when Pakistani security forces stormed the mosque on July 10, 2007, ending a weeklong standoff between military forces and Islamic extremists. Most of those killed in the raid were radical students holed up inside, but the dead also included some women and children.
Government officials say it's too early to tell if this week's blasts in Karachi and Islamabad are reprisal for last year's raid, or payback for a recent military offensive targeting militants in Pakistan's tribal area.
Last weekend, the military launched the operation near Peshawar, the capital of the North-West Frontier Province, in its biggest push against extremists in the tribal region since the civilian government took power in March.
This week's attacks are a crucial test for Pakistan's new civilian government, which is also negotiating with tribal leaders in the lawless border region. CNN's Reza Sayah contributed to this report.
CNN's Reza Sayah contributed to this report.