By Bill Gertz and Eli LakeU.S. intelligence agencies remain on alert but do not think additional package bombs are immediately heading for the U.S. after the third failed attack by the Yemen-based al Qaeda in the Arabian Peninsula over the weekend.

The latest bombing attempts, along with the two earlier failures, show that while the al Qaeda affiliate is continuing to seek international attacks, “operationally, they still aren’t quite there yet,” said a U.S. official familiar with details of the failed attack. The official added, however, that “they are really trying.”

On Sunday, U.S. officials said Western intelligence agencies now suspect that a UPS 747 cargo plane that crashed in Dubai on Sept. 3, killing two crew members, was downed by an explosive package in its cargo bay and not by an onboard fire, as initially suspected. The crash is being re-evaluated in light of the two devices found on transport aircraft in Britain and Dubai … one a UPS jet, the other a FedEx transport.

According to U.S. officials, initial forensic analysis of the bombs recovered in Britain and Dubai indicates the devices contained PETN, a chemical explosive similar to that used in the 2009 body-cavity suicide bomb that exploded close to Saudi antiterrorism chief Prince Mohammed bin Nayef, and in the bomb that failed to detonate in the underwear of attempted bombing suspect Umar Farouk Abdulmutallab aboard a Northwest Airlines flight to Detroit on Dec. 25.

The printer cartridge bombs were destined for Jewish centers in Chicago, although it is not clear how the group planned to detonate the devices once they reached their destination.

John O. Brennan, White House National Security Council staff counterterrorism director, said Sunday that it appears the bombs were intended to blow up the planes in midflight.

“We’re looking at the potential that they would have been detonated en route to those synagogues aboard the aircraft as well as at the destinations,” Mr. Brennan said on CBS “Face the Nation.”

“But at this point we, I think, would agree with the British that it looks as though they were designed to be detonated in flight.”

Mr. Brennan also cautioned, in a separate appearance on “Fox News Sunday,” that the United States “cannot presume no more [explosive devices] are out there”

In his Fox interview, Mr. Brennan confirmed that officials in Yemen have arrested two women “who delivered the packages” to FedEx and UPS offices.

Hanan al-Samawi, a 22-year-old computer-engineering student, and her mother were detained Saturday after the student’s telephone number appeared on one of the packages.

However, according to reports later Sunday from Yemen, police there have released Miss al-Samawi, although it was unclear Sunday evening whether her mother still was detained.

A Yemeni official told the Associated Press on the condition of anonymity that the shipping agent told investigators, upon seeing Miss al-Samawi, that she did not sign the shipping documents. Yemeni authorities now think Miss al-Samawi had her identity stolen by someone who used it to mail the bombs, the official said.

Blowing up transport aircraft is another sign that the Yemen-based al Qaeda affiliate is having difficulty conducting mass-casualty attacks and is focusing on economic targets, analysts say.

Security remains heightened for all aircraft coming from Yemen and especially cargo originating from the southern Arabian Peninsula state.

In London, British Home Secretary Theresa May said security around all international air cargo arriving in Britain was being reviewed. Germany and France have stopped all air freight originating from Yemen as a result of the plot. FedEx and UPS have independently done the same.

“We are looking at the screening of freight. We will be looking at the processes we use. We’ll be talking with the [aviation] industry about these issues,” Ms. May told BBC.

Additionally, Western intelligence agencies suspect the devices were designed and constructed under the direction of Ibrahim Hassan al-Asiri, chief bomb maker for al Qaeda in the Arabian Peninsula. Despite the three failures by al-Asiri to successfully conduct an international terrorist bombing, U.S. and allied intelligence agencies consider him a serious threat.

Mr. Brennan said “the individual who has been making these bombs … is a very dangerous individual, clearly somebody who has a fair amount of training and experience. And we need to find him. We need to bring him to justice as soon as we can.”

The latest bombing attempt was thwarted largely with the help of Saudi Arabia’s intelligence service, known for their ability to penetrate Middle East terrorist groups.

The Saudis on Thursday night provided the key piece of intelligence, warning that two package bombs filled with PETN were heading for the United States, said U.S. officials familiar with the incident.

However, one U.S. official said the package bombs do not appear directly linked to recent terrorist threats in Europe, which also originated from Saudi intelligence.

Sen. Christopher S. Bond of Missouri, ranking Republican on the Senate Select Committee on Intelligence, said he was surprised that the Saudi sourcing for the threat was made public.

“Why do you tip off the fact that the Saudis told us, why tell the source to the world?” Mr. Bond said. “I am confounded by that. I think this gives al Qaeda a game plan for figuring out who is responsible for their problems this weekend.”

Saudi Arabia has closely guarded its intelligence cooperation with the West, fearing it would induce stepped-up retaliation by terrorists. However, U.S. officials said that since the attack on Prince Nayef, Riyadh has declared open war on al Qaeda in the Arabian Peninsula.

Mr. Bond also said in an interview that he thinks there will be “a wave of attempts.”

“It is obvious [the al Qaeda terrorists] are making an effort” to attack, he said. “We need to do a much better job in screening. Any planes in the United States … we need to make sure we have it clear on where or who is doing what, are they making sure packages are safe either in a cargo plane or a passenger plane. I understand, at some point these [packages] traveled on a passenger plane.”

Bill Cowan, a retired special operations officer, said it is possible that the Arabian Peninsula terrorist group is seeking to send explosives for attacks into the United States and have someone here construct the bomb, something that will require better U.S. human intelligence gathering.

Mr. Cowan also questioned whether the Saudis were the source of the tip.

“If so, why alert AQAP that they have a leak from some connection with the Saudis?” he asked, noting that it is more likely that the real source of the tip came from somewhere else.

“Despite the vast amounts of money spent on technology, we see again that it’s basic human intelligence that comes through,” Mr. Cowan said. “Unfortunately our intelligence services are not where they should be on the ‘hum-int’ side. We have to rely on our allies.”

• Joseph Weber contributed to this report.