US authorities are investigating whether al-Qaeda was involved in a failed attempt to blow up a US airliner on Christmas Day, the head of the country's homeland security has said.
Janet Napolitano, however, told US television broadcasters on Sunday that there was no early evidence to suggest that the 23-year-old Nigerian suspect in the case was part of a larger al-Qaeda plot.
"Right now, we have no indication that it is part of anything larger," Napolitano told CNN's "State of the Union" programme.
"But obviously the investigation continues. We have instituted more screening and what we call mitigation measures at airports."
A US federal judge formally charged the suspect, identified as Umar Abdulmutallab, on Saturday, at the University of Michigan Medical Centre in Ann Arbor, where he was being treated for burns.
The US Federal Bureau of Investigation (FBI) said that preliminary analysis had shown that the suspect had strapped pentaerythritol, which he tried to ignite, to his body.
Transportation officials said another US passenger plane was involved in an emergency incident on its approach to the central US city of Detroit on Sunday.
Crew members of Northwest Airlines flight 253 from Amsterdam requested emergency assistance upon landing after reporting a "disruptive" passenger, the US Transportation Security Administration (TSA) said.
Officials said the plane landed safely and the passenger was taken into custody at the Detroit airport.
But the US news channel CNN, citing law enforcement officials, later reported that the detained passenger did not appear to be a threat.
Al Jazeera's John Terrett, reporting from Washington, said Sunday's incident involved the same flight number as the Christmas Day flight.
"At the moment there is nothing to suggest this is anything more than an extraordinary coincidence," he said.
"But nonetheless, an extraordinary coincidence that something similar could happen so soon after the incident on Christmas Day."
US media cited officials saying the suspect in Friday's incident had told interrogators that al-Qaeda operatives in Yemen had given him the equipment and instructions on how to carry out the attack.
Abdulmutallab was on the US Terrorist Identities Datamart Environment (TIDE) security watch list, which includes about 550,000 people.
|Abdulmutallab is the son of a prominent Nigerian banker [Reuters]
However, the Nigerian was not on the smaller Terrorist Screening Data Base (TSDB) and was not flagged for mandatory extra airport security screening or included on the "no fly" list.
US authorities told The Associated Press news agency that the suspect came to the attention of intelligence officials in November when his father went to the
US embassy in the Nigerian capital, Abuja, to express concerns about his son.
Al Jazeera's Yvonne Ndege, reporting from Abuja, said Nigerian authorities had launched an investigation, concentrating on how the suspect breached security and boarded the aircraft.
He is believed to have started his journey in Lagos before the flight stopped at Amsterdam's Schiphol airport on the way to Detroit.
Ndege said that Abdulmutallab's family had been called to the capital for questioning, but that there was no suggestion that they were involved in illegal activities.
"There's been shock, condemnation and disbelieve from millions of Nigerians ... They simply cannot believe that the son of a prominent banker ... from a very privileged family, somebody who was educated privately in some of the world's best educational institution could be involved in this kind of event."
Passengers travelling on Saturday and Sunday felt the consequences of the attempted attack.
Several airlines told their passengers that new US regulations prevented them from leaving their seats from an hour before landing.
Douglas Laird, an aviation security analyst, told Al Jazeera that technology has to be improved to ensure safety onboard aircraft.
"Schiphol [Amsterdam] is one of the best airports in Europe in a security sense, very very efficient, very thorough," he said.
"The problem is... if you want to find what people are hiding on their bodies and bringing on to the airplane, you need to do a body scan.
"But there's no magic bullet. There's no one thing that is going to make flying 100 per cent safe. What you want to do is to cover many bases and overlap the security measures. Hopefully that will cover what you are looking for."