Washington ? President Bush said Tuesday that Jose Padilla, the man arrested in an alleged plot to spread radioactive material across parts of America, is one of many “would-be killers” the United States has in custody. And there will be more, he promised.

“This guy Padilla’s one of many who we’ve arrested,” Bush said in a Cabinet Room meeting on his proposed overhaul of homeland security programs.

“The coalition we’ve put together has hauled in 2,400 people. And you can call it 2,401 now. There’s just a full-scale manhunt on. … We will run down every lead, every hint. This guy Padilla’s a bad guy and he is where he needs to be, detained.”

Bush spoke about Padilla the morning after his May 8 was announced in Washington.

In Budapest, Attorney General John Ashcroft said Tuesday the indefinite

imprisonment of Padilla, 31, is proper because of the seriousness of the threat.

“We believe that by his detention that we have significantly disrupted a potential plot to deploy a dirty bomb, an explosive device, in the United States,” Ashcroft said.

Defending the decision to consider Padilla, also known as Abdullah al Muhajir, an enemy combatant, Ashcroft said the government has “very significant information” about Padilla’s involvement “with al-Qaida in very serious terrorist plots.”

The government, which on Monday revealed Padilla’s arrest, described the move as a significant blow against an al-Qaida plot to detonate a radiological weapon known as a “dirty bomb” inside the United States, possibly targeting Washington. Authorities described Padilla as a former gang member from Chicago who was raised Catholic but converted to Islam.

Officials said the plot got only as far as the planning stage and they said there was no indication Padilla had access to nuclear materials. Undersecretary of State John Bollton indicated Padilla was carrying plans for the rthe attack when he was picked up in Chicago.

Already on the trail of an alleged plot to spread deadly radioactive material across parts of America, U.S. investigators in Pakistan watched Padilla, their key suspect, board a plane bound for Switzerland.

Padilla believed he was slipping away from Pakistani authorities who had detained him and at least two colleagues on immigration charges, a U.S. official said. Authorities suspected Padilla, a U.S. citizen, was headed to America to scout locations for possible bombings.

But Padilla, traveling from Pakistan via Zurich to Chicago’s O’Hare International Airport, was under continuous surveillance by U.S. agents on those flights, and the FBI was waiting to arrest him May 8 as his plane arrived at the gate.

Dirty bombs comprise traditional explosives combined with radioactive material. They would not create a nuclear explosion, but they could release small amounts of radioactive material over dozens of city blocks. Experts believe the most devastating effect would be the ensuing panic and the difficulty sending rescue workers into the contaminated area.

In an unusual legal twist, the Justice Department handed over the Brooklyn-born Padilla to the Pentagon for indefinite imprisonment as an “enemy combatant.” Government lawyers cited a 1942 Supreme Court ruling permitting such a transfer. Padilla had been held quietly for weeks in New York City, then was flown Monday aboard a military C-130 plane to a Navy brig in Charleston, S.C.

“We have acted with legal authority both under the laws of war and clear Supreme Court precedent, which establish that the military may detain a United States citizen who has joined the enemy and has entered our country to carry out hostile acts,” Ashcroft said.

In a statement attributed to al-Qaida spokesman Sulaiman Abu Ghaith, the group said: “We have the right to fight (Americans) by chemical and biological weapons so that they catch the fatal and unusual diseases that Muslims have caught due to their chemical and biological weapons.”

Ashcroft, who first disclosed the arrest in a television announcement from Moscow, said Padilla “trained with the enemy,” studying how to wire explosives and researching radiological weapons. Ashcroft said Padilla met several times in 2001 with senior al-Qaida officials in Pakistan and Afghanistan, where he traveled after he served one year’s probation on state weapons and assault charges in Sunrise, Fla.

Information leading to Padilla’s arrest came in part from U.S. questioning of captured al-Qaida leader Abu Zubaydah, one of Osama bin Laden’s top lieutenants, two U.S. officials said. Ashcroft said information about the plot came from “multiple independent and corroborating sources.”

Padilla first met Abu Zubaydah in Afghanistan in late 2001 after the Sept. 11 terror attacks, then went to Lahore, Pakistan, to research dirty bomb techniques with an unidentified associate, officials said. At Abu Zubaydah’s request, Padilla traveled to Karachi, Pakistan, in March to meet several senior al-Qaida officials and discuss bombings of U.S. gas stations and hotels, officials said.

Padilla was described by one former neighbor in Chicago as “so quiet, so nice.” Nicknamed “Pucho” as a teen-ager, he enjoyed basketball and video games with his friends.

Padilla was convicted at 15 as a juvenile of aggravated battery, armed robbery and attempted armed robbery. A law enforcement official said he was in custody there between November 1985 and May 1988.

In Florida, he was convicted in 1991 in Sunrise on charges of aggravated assault and discharging a firearm, court records show. Padilla, who identified himself as Catholic when he was booked on those charges, served one year of supervised release, until Aug. 4, 1993.

Police said Padilla had brandished a .38-caliber revolver at another driver during a traffic encounter and later fired a shot from his car. No one was injured.

While in the Broward County jail, Padilla was accused of battery on a jail officer and resisting without violence in January 1992. He pleaded guilty and spent 10 months behind bars.