Opinion by Judge ALARCÓN; Concurrence by Judge JAMES R. BROWNING.

The Confederated Salish and Kootenai Tribes of the Flathead Indian Reservation (the “Tribes”) appeal from the order granting summary judgment in favor of Secretary of the Interior, Gale Norton (the “Secretary”). The Tribes sought a declaration that, upon the Tribes’ request, the Secretary is required to take certain land in trust for the Tribes or the tribal member to whom the land is sold pursuant to the Act of July 18, 1968, Pub.L. 90-402, 82 Stat. 356 (the “Flathead Act”). We affirm because we conclude that the Flathead Act authorizes the Secretary of the Interior to exercise his or her discretion in acting upon tribal requests for land acquisitions within the reservation boundaries.

I

The relevant portions of the Flathead Act provide as follows:

Act of July 18, 1968, Pub.L. No. 90-402, 82 Stat. 356 (emphasis added).

On June 2, 1999, the Tribes requested that the Department of the Interior place in trust a forty-acre parcel of grazing land, within the Flathead Indian Reservation, which the Tribes owned in fee. In an undated decision received by the Tribes on July 9, 1999, the Bureau of Indian Affairs (“BIA”) Superintendent, acting on behalf of the Secretary, approved the Tribes’ request. The Superintendent’s decision is not part of the record on appeal. The parties agree that in reaching his decision, the BIA Superintendent referred to some of the criteria set forth in 25 C.F.R. § 151.10 . 1 Section 151.10 requires the Sec *1195 retary to provide notice to state and local governments before approving discretionary land acquisitions. The Secretary need not provide such notice under § 151.10, however, if Congress has mandated the land acquisition. The Superintendent did not provide notice to state and local governments.

In face of the Superintendent’s decision to grant the Tribes’ request and take the forty-acre tract in trust, the Tribes, nevertheless, appealed from the decision to the Northwest Regional Director of the BIA because they were concerned that the Superintendent’s reliance on § 151.10 indicated that he believed that § 2 of the Flathead Act was discretionary. The Regional Director held that the Superintendent had the discretion to grant or deny the Tribe’s request. He vacated the decision of the BIA Superintendent because of the Superintendent’s failure to give notice to local and state governments. The Regional Director reasoned as follows:

The Tribes appealed from the Regional Director’s decision to the Interior Board of Indian Appeals (“IBIA”). The IBIA affirmed the decision of the Regional Director. The IBIA ruled that, under § 2 of the Flathead Act, the Secretary had discretionary authority to acquire land upon the Tribes’ request. Confederated Salish and Kootenai Tribes of the Flathead Nation v. Northwest Reg’l Dir., Bureau of Indian Affairs, 35 IBIA 226, 232 (Nov. 9, 2000). In so holding, the IBIA did not address the Superintendent’s failure to give notice to state and local governments.

II

The Tribes filed this declaratory judgment action in the United States District Court for the District of Montana. The Tribes requested that the district court declare that the Flathead Act “is a mandatory fee-to-trust statute” and “that the Secretary ... must put title to Tribally-owned Reservation lands into trust upon the request of the Tribes, free from the participation of local governments.... ” The district court granted the Secretary’s motion for summary judgment. The dis *1196 trict court held that § 2 of the Flathead Act authorized the Secretary to grant or deny the Tribes’ request. The district court had jurisdiction under 28 U.S.C. §§ 1331 , 1362 and 2202. We have jurisdiction over the Tribes’ timely appeal under 28 U.S.C. § 1291 .

Ill

The Tribes contend that the Flathead Act unambiguously mandates that the Secretary acquire land and place it in trust status upon the Tribes’ request. “We review de novo a grant of summary judgment. We also review de novo questions of statutory interpretation.” Bedroc, L.L.C. v. United States, 314 F.3d 1080 , 1083 (9th Cir.2002) (internal citations omitted).

“We interpret a federal statute by ascertaining the intent of Congress and by giving effect to its legislative will.” Ariz. Appetito’s Stores, Inc. v. Paradise Village Inv. Co. (In re Ariz. Appetito’s Stores, Inc.), 893 F.2d 216, 219 (9th Cir.1990). In determining Congress’s intent, we must first look to the words of the statute. Conn. Nat. Bank v. Germain, 503 U.S. 249, 253-54 , 112 S.Ct. 1146 , 117 L.Ed.2d 391 (1992). “When the words of a statute are unambiguous ... judicial inquiry is complete.” Id. at 254 , 112 S.Ct. 1146 (internal quotation marks and citations omitted). “Where the language is not dis-positive, we look to the congressional intent revealed in the history and purposes of the statutory scheme.” United States v. Buckland, 289 F.3d 558, 565 (9th Cir.2002) (en banc) (citation and internal quotation marks omitted).

The Secretary maintains that “[t]he plain language of the Flathead Act authorizes but does not require the Secretary of the Interior to act on Tribal requests for land acquisitions.” We agree. Under § 2 of the Flathead Act, the Secretary is authorized, upon request, to acquire Indian land within the reservation boundaries. The word “authorize” is defined as follows: “to endow with authority or effective legal power, warrant, or right.” Webster’s Third New International Dictionary 146 (4th ed.1976). “Authority,” in turn, means “superiority derived from a status that carries with it the right to command and give final decisions.” Id. The ability to exercise power implies discretion. Black’s Law Dictionary states that “to authorize” means “to give legal authority; to empower.” Black’s Law Dictionary 129 (7th ed.1999). “Power” is defined as “[t]he legal right or authorization to act or not act.” Id. at 1189 (7th ed.1999) (emphasis added). Another legal lexicon defines “authorize” as having “the connotation [of] being permissive, rather than mandatory.” Ballentine’s Law Dictionary 112 (3d ed.1969). Ballentine’s also states that where “authorize” is construed to imply a mandate, it is because other words “have been used to express that intention.” Id.

In § 1 and § 2 of the Flathead Act, Congress used the word “shall” to describe what the Secretary must do if he or she decides to honor a tribe’s request to dispose of or acquire land. Section 1 expressly provides that the net proceeds from tribal lands disposed of by the Secretary of the Interior in the exercise of his or her authority “shall be used to acquire ... additional lands.” Section 2 provides that title to lands acquired by the Secretary of the Interior in the exercise of his or her authority “shall be taken in the name of the United States in trust....” Thus, Congress clearly demonstrated in the Flathead Act that it knows how to use the word “shall” when it wishes to mandate an act. Congress’s use of the term “authorized” rather than “shall” in setting forth the Secretary’s powers to dispose of or acquire land upon tribal request demonstrates its intent that the Secretary of the Interior should exercise his or her judgment in determining whether it is in a *1197 tribe’s best interests to honor its request to dispose of or acquire land.

Because we are persuaded that the Flathead Act is unambiguous, our “judicial inquiry is complete.” Conn. Nat. Bank, 503 U.S. at 254 , 112 S.Ct. 1146 . We lack the authority to attempt to divine Congressional intent by looking to legislative history or comparing the language used in other statutes. Therefore, we hold that the Flathead Act authorizes the Secretary of the Interior to exercise his or her discretion in determining whether to grant a tribe’s request to acquire land within the reservation boundaries under § 2.

AFFIRMED.

. 28 C.F.R. § 151.10 provides:

(Emphasis added).