Why attack Wagah crossing on India-Pakistan border?
As at least 55 people are killed in a suicide bombing at the Wagah crossing on the India-Pakistan border, the BBC's Sanjoy Majumder in Delhi looks at why the target is so significant.
The Wagah border is the only land crossing between India and Pakistan and the daily flag-lowering "retreat" ceremony is a major tourist draw.
The colourful ceremony takes place at sunset every day, with large, enthusiastic and patriotic crowds ensconced in galleries on both sides of the border.
Goosestepping border guards in full ceremonial uniform march and confront each other in dramatic style as the two flags are brought down.
A salute and handshake is followed by the gates on either side being slammed shut.
So the suicide attack on the Pakistani side at Wagah has raised concerns not just because it happened at a highly secure area, but also because it took place in Punjab, Pakistan's most populous and prosperous province which is also the home of the country's Prime Minister Nawaz Sharif.
Tens of thousands of people visit Wagah on most days and have to pass through layers of security.
The border itself has been on a state of heightened security in recent weeks following artillery exchanges between India and Pakistan towards the north.
But this latest attack has only raised fresh concerns.
Security has been stepped up on the Indian side with border officials saying a red alert has been sounded.
The ceremony itself was originally going to be put off for three days, following a request from the Pakistani Rangers, who secure the border on the Pakistani side.
But in the end Monday's ceremony went ahead, with Pakistan's military saying it wanted the nation to unite against terrorism.
The only other times the ceremony has been interrupted were during the 1965 and 1971 wars between India and Pakistan.
Reports in the Indian media quote unnamed government sources as suggesting that the country's intelligence service had some prior warning of a possible attack.
"The warning... came after Pakistan's Rangers enhanced their forces and were seen erecting new defensive structures," the Indian Express reports.
India's Research and Analysis Wing, the country's main spy agency, also apparently detected preparations for a suicide attack, the newspaper said.
Some analysts are now warning that India needs to worry about the security situation on its side of the border.
It is not clear if the suicide bomber meant to target people on both sides.
But what has raised concern here is the fact that he was able to get as close to the border checkpoint as he did.
"It's a heavily secured area, very difficult for anyone to breach," says Commodore Uday Bhaskar, director of the Society for Policy Studies think-tank in Delhi.
"We need to build greater capacity for surveillance both within India and across the border," he adds.
That may be harder to do given the level of distrust between India and Pakistan, whose relations are not at their best at the moment following the recent border firing and the breakdown of peace talks.