Orlando, Fla. ? Stepped-up inspections were ordered Friday for each of the nation’s 7,000 commercial airliners after several box cutters and other contraband material slipped past security and onto two Southwest Airlines jets.

The box cutters — the same type of utility knife thought to have been used by the Sept. 11, 2001, terrorists — were found Thursday night stuffed into plastic bags in lavatory compartments of the planes, according to a statement by the airline.

The bags also contained bleach disguised in suntan-lotion bottles, plasticine modeling clay and notes bragging about slipping the material past security. The modeling clay apparently was included in the packages to mimic plastic explosives, authorities said.

A 20-year-old North Carolina man was being questioned Friday night by the FBI in connection with the incidents, according to a congressional source and a senior law-enforcement official, both of whom spoke on condition of anonymity. The student, the sources said, is believed to have placed packages on up to six Southwest planes, although only two have been found.

It’s unclear when the packages were left on the planes.

The student apparently had been sending letters to the federal Transportation Security Administration since the spring, alerting the agency that he planned to expose holes in the air-travel security system. Last month, just before Hurricane Isabel struck, the congressional source said that the student again warned he was about to bring the bags onto planes.

In essence, the source said, “he gave the TSA advance notice, and they still didn’t catch him.”

The material was found by maintenance crews after the Orlando-to-New Orleans and Austin-to-Houston flights reached their destinations.

The Department of Homeland Security and the TSA immediately ordered all airlines and federal agents to inspect the passenger cabin of every commercial airliner within 24 hours.

But officials also offered assurances to passengers that there was no danger presented by the incident and no evidence of terrorist involvement. Rather, they said, it appeared to have been intended as a slap in the face of the TSA, the agency responsible for security at the nation’s 429 commercial airports.

“Right now, there is basically no evidence to suggest that this is related to terrorism,” said Brian Roehrkasse, a Homeland Security spokesman. “However, the FBI, with the Department of Homeland Security, is aggressively focusing its efforts to determine how these items got on board the aircraft.”

Ed Cogswell, a spokesman for the FBI, said investigators are still trying to figure out what kind of security breach they’re dealing with.

Several aviation experts said Friday that they were not surprised by the incident, saying that no system is going to be 100 percent effective.

“You are not going to catch everything,” said David Stempler, president of the Air Travelers Association.

With that in mind, Stempler and others said they doubted that this recent incident would result in any major change in the way TSA handles airport security.

“They are dancing as fast as they can, but they’ve got a lot of holes to plug,” he said.

While agreeing that the incident highlights one of the system’s shortcomings, Rep. John Mica, R-Fla., chairman of the House Transportation Committee’s aviation subcommittee, said there are other elements of aviation security designed to deal with such failures.

“We do have a system of redundancies that, hopefully, would help,” Mica said.

Those include reinforced cockpit doors, armed federal marshals on some flights and pilots who can carry guns.

“But if people are willing to carry explosives on board, we don’t have a system that protects against that,” Mica said.

Mica, who led a hearing on Thursday with TSA head James Loy, said he has been disappointed in the pace of the effort to screen all passenger baggage for explosives. The TSA also needs to move faster to obtain better technology than the X-ray systems now in use, he said.

The TSA is working on new training programs for baggage screeners and enhanced detection systems, Mica said, but progress has been agonizingly slow.

“Maybe this will help speed it up,” he said.