Prime Minister Imran Khan in a tweet on Monday "assured the nation" that those [found] guilty in the Sahiwal incident would be given an "exemplary punishment", adding that the grief and anger of the people of Pakistan over the incident was "understandable and justified".

"I assure the nation that when I return from Qatar not only will the guilty be given exemplary punishment but I will review the entire structure of Punjab police and start [the] process of reforming it," added the premier, who departed for Doha today for a two-day official trip.

Human Rights Minister Shireen Mazari also took to Twitter, calling for an "end [to] decades of tolerance for killings through 'encounters'".

"What was tolerated, even encouraged by prev governments/state must end now as we strengthen Rule of Law and accountability for all," the minister said.

The incident is being probed by a joint investigation team (JIT) which is in Sahiwal to examine the crime scene, sources in the CTD said. The JIT will also interrogate eyewitnesses and local police as well as the CTD officials involved in the incident. According to a first information report filed earlier, the team involved in the incident was led by CTD Sub-Inspector Safdar Hussain, and included corporals Ahsan, Ramzan, Saifullah and Hasnain.

Video footage of the incident, statements of the surviving children, and initial statements by the CTD officials will also be included in the investigation, the sources added.

The JIT will present its report to the Punjab chief minister on Tuesday.

On Saturday, in what law enforcers described as an encounter with terrorists, the elite Punjab police killed four people, including a couple and their 13-year-old daughter, sending shock waves across the country as one of the three surviving children who were witness to the episode denied the official version in a video that went viral on social media.

A day after the incident, Prime Minister Khan had promised that "swift action" would be taken against those found responsible for wrongdoing.

As the funeral prayers were held for Khalil, his wife Nabeela, their daughter Areeba, and neighbour Zeeshan on Sunday evening, the Counter Terrorism Department (CTD) said it was actually following Zeeshan — who was behind the wheel of the car that was showered with bullets — and regretted the killing of the couple and their daughter.

The CTD said it was following Zeeshan because he was a facilitator of terrorists. It claimed that two terrorists, Kashif and Abdul Rahman, who were killed in an encounter with the CTD in Gujranwala later on Saturday night, were Zeeshan’s accomplices.

Sahiwal police on Sunday registered a case under murder and terrorism charges against 16 unidentified officials of the CTD on the complaint of Khalil’s brother Muhammad Jalil.

According to the initial post-mortem report, Khalil was hit by 13 bullets, Nabeela four, Areeba six and Zeeshan 10, in the head, shoulder and chest. The bullets were fired at point-blank range.

The CTD, on the other hand, registered a case under attempt to murder, murder, terrorism and other charges against the “suspected terrorists”.

Wait for the JIT report: Punjab CM

Punjab Chief Minister Usman Buzdar said he will receive the JIT's preliminary report into the incident tomorrow, when the 72 hour-time period given to the probe team ends at 5pm. He advised the media against assigning the blame on anyone until the report is released.

"Do not say anything to anyone until 72 hours [have ended] ... wait for the report," he told reporters in Lahore, adding that he will review the report at a meeting on Tuesday.

He assured the nation that they will see a "clear difference" between how the JIT report is acted upon "in Naya Pakistan" and how JIT reports were treated during previous governments' tenures.

"Once the preliminary report is out, you can question us on what action we have taken [over the incident]," he told a media representative.

Opposition protests 'cold-blooded' killings

Also on Monday, the opposition in the Senate condemned the killings in the Sahiwal 'encounter' and rejected the government's stance on the incident.

Speaking during the session, PPP Senator Sherry Rehman said the authorities were constantly changing their stance on the "cold-blooded" killings but that they were standing against visual evidence.

"This is an age of transparency when the camera captures everything," she said, stressing that "extrajudicial killings" should come to an end.

She questioned the motive behind the formation of a JIT to probe the incident, saying such teams had not been formed over similar incidents in the past.

Rehman also asked that if the deceased were suspected terrorists, "Was there no other option than to open fire on them?"

She said the people are now wondering whether they should leave their homes with their children. "Who is safe in Pakistan?" she asked.

She said the incident smelled of a "cover-up" and sought a government response on the matter by tomorrow.

Jamaat-i-Islami chief Sirajul Haq said those who gave "wrong information" regarding the deceased were more culpable than those who opened the fire. "Which intelligence departments were these who gave the false alert?" he questioned.

"Even if they were terrorists, do the agencies have the right to fire bullets like this on the roads?" he said, wondering whether the country's courts should then be padlocked.

The senator demanded that an audit be conducted of all police shootouts in the country.

Chairman Sadiq Sanjrani later directed the Senate Standing Committee on Interior to "immediately" conduct an inquiry of its own into the Sahiwal incident.

He directed the committee to submit a comprehensive report within a week, assigning responsibility for the killings. The body has also been tasked with giving recommendations to prevent similar episodes in the future and present a plan for the guardianship of the children whose parents were killed in the Sahiwal firing.