Story highlights Polio workers shot dead on Sunday in northwest Pakistan

Female suicide bomber attacks bus with women on it, source tells CNN

Pakistani official thinks "homegrown terrorists" may be responsible

Officer dies in bombing of Pakistan founder's home

Militants, including a female suicide bomber, attacked a university bus carrying women in southwestern Pakistan and then struck a hospital where the survivors were taken for treatment, authorities said Saturday.

The militants laid siege to the Bolan medical complex in Quetta, holding hundreds of patients, physicians and nurses hostage as they battled security forces, authorities said.

An intelligence official, who requested anonymity since he was not authorized to speak to media, told CNN a female suicide bomber carried out the bus attack.

Twenty-eight people, including four militants, were killed in the siege at the hospital. The gunmen killed four security force members, a medical administrator, and the deputy commissioner of Quetta. Four nurses died in the crossfire, police said. Two militants blew themselves up, and two more died in a shootout, authorities said.

Among the hospital patients were more than 20 people wounded in the university bus explosion, which killed at least 14 women, Interior Minister Chaudhry Nisar Ali Khan said.

Several others were wounded in another blast tphat struck near the hospital's emergency room as the bus bombing victims were rushed in.

Photos: Photos: Bus bomb, hostages in Pakistan Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – Pakistani paramilitary soldiers take positions after militants attacked a hospital in Quetta, Pakistan, on Saturday, June 15. Several militants held hundreds of people hostage inside the Bolan medical complex. Hide Caption 1 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – Pakistani soldiers arrest a suspected militant at the hospital. Among the patients at the complex were people wounded in a bus bombing near Sardar Bahadur Khan Women University that killed 12 female students. Hide Caption 2 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – A man carries an injured boy after the hospital attack. Seven people have killed in the siege of the hospital. Hide Caption 3 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – Soldiers take position near the hospital. Hide Caption 4 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – A man carries an injured boy away from the scene. Hide Caption 5 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – Soldiers shift an injured colleague into a van. Hide Caption 6 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – People exit the hospital in Quetta. Hide Caption 7 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – Firefighters extinguish a burning university bus after the bomb blast in Quetta. Hide Caption 8 of 9 Photos: Photos: Bus bomb, hostages in Pakistan Bus bomb, hostage situation in Pakistan – Firefighters work to extinguish the burning bus. "All the (bus) victims are women teachers and students," according to a police official, Mir Zubair Mehmood, in Quetta. Hide Caption 9 of 9

"All the (bus) victims are women teachers and students," said Mir Zubair Mehmood, a police official in Quetta. The blast shattered the windows of offices and classrooms inside Sardar Bahadur Khan Women University.

The university bus was parked at the school on the outskirts of Quetta, the capital of Balochistan province in southwestern Pakistan.

There was no immediate claim of responsibility. But Pervez Rasheed, the country's information minister, said, "We do not think there is any foreign involvement in these attacks -- this looks like homegrown terrorists."

U.N. Secretary-General Ban Ki-moon condemned the attacks, saying "violence against women and educators has increased in recent years. The aim being to keep girls from attaining a basic right to education."

Ban called on Pakistan's new leader, Nawaz Sharif, to do "all possible to bring the perpetrators to justice."

On Sunday, gunmen killed two polio workers who were making home visits in Khyber Pakhtunkhwa province in northwestern Pakistan, police said. Sharif called the attack "cowardly and inhumane."

The attack occurred in Ziarat, one of the country's top tourist spots, about 150 kilometers (93 miles) from Quetta. Jinnah spent the last days of his life at the home.

Islamabad summons a top U.S. envoy over deadly zone strike